Test Firefox Relay Email Deliverability

Why Firefox Relay masked addresses fail deliverability testing

You send a campaign to an email list. The results show high bounce rates — not because of invalid addresses, but because many are Firefox Relay aliases. You verify them. They pass every check. Yet they still don’t land in inboxes. Why?

Firefox Relay creates disposable aliases that forward to real inboxes but aren’t the actual destination. Tools that test deliverability assume they’re checking the final delivery point. When they can’t inspect the real inbox behind the proxy, they mark the alias as unverifiable or suspicious — even if the underlying email is valid.

This isn’t a flaw in your list. It’s a flaw in how most verification tools handle forwarding layers. Standard checks can’t see beyond the alias to the real destination. The result? False negatives, wasted sends, and poor inbox placement for real users.

Key takeaways

Firefox Relay aliases are proxies, not final delivery points, so standard deliverability tests fail by design.
Verification tools that can’t inspect forwarding infrastructure return false negatives on Relay addresses.
Testing deliverability for Relay masked addresses requires inbox placement checks, not just syntax or MX validation.

Can you test email deliverability for Firefox Relay emails?

You can test email deliverability for Firefox Relay masked addresses — but only with tools that simulate real sending behavior and validate end-to-end inbox delivery, not just syntax or MX records. Relay aliases act as forwarders, so verification must confirm that messages sent to the alias actually reach the real inbox, not just that the address format is valid. Tools that stop at DNS checks or syntax parsing will fail, since they can’t detect forwarding logic or user intent.

Why standard validation fails with Relay addresses

Firefox Relay masks your real email by routing messages through a proxy. A simple syntax check or MX record lookup only confirms the alias exists — not that it forwards correctly. Many tools stop there, giving a false sense of security. This is why relying on basic validation is misleading: your message might bounce or vanish in the forward chain, even if the Relay address is technically "valid."

How real deliverability testing works

True deliverability testing sends a real message to the Relay alias and traces it through the entire delivery path — from sender to final inbox. The tool must simulate actual SMTP behavior, including proper headers, TLS, and reputation signals. Only then can it confirm if the email lands in the recipient’s inbox, spam folder, or gets blocked entirely. This approach aligns with industry standards like those outlined in RFC 5321 for SMTP delivery and RFC 5322 for message format.

For example, tools that test deliverability through actual email sending — not just static checks — are better suited for Relay testing. They help you catch issues like throttling, rate limits, or filtering by the forwarder service itself. The goal isn’t just to verify the address exists, but to confirm that your content gets through as intended.

Use a platform like inbox placement testing to evaluate how messages to Relay addresses perform across real inboxes. This includes monitoring real-time delivery status, spam detection, and folder placement — giving you actionable insights beyond a simple “valid” or “invalid” flag. It’s the only way to validate deliverability in ecosystems like Firefox Relay that rely on forwarding, not direct delivery.

How Emaillistchecker.io tests deliverability for Firefox Relay addresses

You can test deliverability for Firefox Relay masked addresses by sending a real email via SMTP and confirming if it reaches the inbox — not just the mailbox, but the final inbox. Our inbox-placement testing simulates real sending conditions across major email providers, including those handling Relay aliases. This tells you whether masked addresses are actively receiving mail, avoiding false negatives from temporary or blocked Relay endpoints.

Real SMTP delivery, not just syntax checks

Many tools stop at checking if an email has a valid format or domain. We go further: we send a real message using SMTP, just like your email service would. This catches issues like enforced greylisting, Relay-specific filtering, or temporary blocking that syntax-only checks miss.

When you test a Firefox Relay address with us, the message is delivered to the actual mail server — not just validated at the domain level. This means we can verify if the address is still active and accepting messages, even if it’s been recently created or masked.

For example, Firefox Relay uses aliases that route to real inboxes. Some of these aliases may be delayed due to temporary server throttling. Our method detects that delay by monitoring actual delivery, not just DNS records. This is a key difference from tools that rely on DNS or email pattern matching alone.

What this means for your list health

Using a real SMTP send gives you accurate insight into whether you can reliably reach users with masked addresses. If a Relay alias is blocked or paused, it won’t accept the message — and our test will show that.

This is especially useful for sending to users who opt into privacy-preserving tools. You're not just testing if an email exists — you're testing if it can *receive* mail when used as a Relay alias.

Unlike some validators that flag Relay addresses as “risky” or “invalid” due to their nature, we treat them as active if they accept a real email. This prevents false positives that could lead to losing real customers.

For more context, the IETF’s RFC 6531 outlines how modern email systems handle internationalized addresses and delivery semantics — including the idea that delivery success is best measured by actual transmission and final receipt, not just syntax. We align with that principle.

Learn how to run inbox-placement tests at scale with our inbox-placement tool or integrate real-time verification into your workflow with our API. Start with 100 free verifications at our pricing page.

Step-by-step: How to test a Firefox Relay address with Emaillistchecker.io

You can test if a Firefox Relay masked email address receives messages by running an inbox placement test on Emaillistchecker.io. Enter the Relay address directly—like [email protected]—click 'Test Deliverability', and get results in 1–3 minutes. The outcome shows whether the email was delivered, bounced, or blocked. This is the fastest way to verify that Relay users can actually receive mail without compromising privacy.

How the test works

Firefox Relay masks your real email with a disposable one, forwarding messages to your actual inbox. It’s designed for privacy, not deliverability testing. But if you're sending to Relay addresses, you need to confirm they're viable. Emaillistchecker.io simulates a real send using the same mechanisms email systems use to evaluate new senders, including checking for spam signals, authentication, and server behavior.

Go to the inbox-placement tester at Emaillistchecker.io/inbox-placement. No signup is required. This tool is built to assess how real-world email providers treat your sender profile.
Enter the Firefox Relay email address, such as [email protected]. Relay addresses are structured to be unique per user, and the system treats them like any other email for routing purposes.
Click 'Test Deliverability'. The system will initiate a secure, automated test using real email infrastructure. There's no risk to your sender reputation.
Wait 1–3 minutes. During this time, the system checks DNS records, SPF/DKIM alignment, server response times, and spam filter behavior—same as major providers like Gmail, Outlook, and Yahoo. These checks are aligned with standard SMTP behavior [RFC 5321].
Review the result. 'Delivered' means the email reached the user's inbox. 'Blocked' or 'Bounced' indicates the server rejected it—common for overly aggressive spam filters, missing authentication, or Relay-specific rate-limiting.

What to do with the result

If the test fails, your content or sending pattern may trigger filters. This doesn't mean Relay is broken—just that your message may be flagged. Use the bulk verification tool to catch similar issues at scale. If you're sending to many Relay users, consider using a dedicated sender domain and proper authentication (SPF, DKIM, DMARC). These are industry-standard practices to reduce the chance of delivery failure [Spamhaus].

What the deliverability test means for your email list

Testing email deliverability for Firefox Relay masked addresses tells you whether those addresses are actually receiving mail. A "Delivered" result means the address is active and properly configured to accept messages. A "Blocked" or "Bounced" result reveals problems—either with your sender reputation, infrastructure, or the forwarding setup—so you can fix or remove the bad entries before sending. This helps maintain list health and prevents wasted sends.

Understanding deliverability outcomes

When a Relay address shows as Delivered, it means the forward configuration is correct and the underlying inbox is accepting messages. This is a green light to include such an address in your campaigns—assuming you’re compliant with privacy standards.

A Blocked result typically means something in your send setup is triggering defensive systems. It could be your sending IP on a blocklist, a domain flagged for spam, or content that looks suspicious to filters. You can check blocklist status using tools like Spamhaus or MxToolbox. If your sender infrastructure is clean, the issue might be content-based—like links or wording that triggers spam engines.

A Bounced result suggests the forward chain failed. This may mean the original mailbox is unreachable, the Relay provider’s forward system isn’t working, or the receiving server is rejecting the message at the SMTP level. Unlike a simple invalid address, a bounce here reflects a server-side problem. You won’t know if it’s the sender, the forwarder, or the final inbox—not all bounces are equal.

Use test results to clean your list

Every test result should inform your next step. If you're sending to Relay addresses and some are bouncing or blocked consistently, that’s a signal to review how your emails are being sent. You might need to adjust your sender IP reputation, use an authenticated service, or avoid aggressive content.

Filter out addresses that repeatedly fail deliverability checks—especially if they're Blocked or Bounced. Keeping them on your list hurts sender reputation and increases the chance of future emails being deprioritized or quarantined.

Use verified, deliverable addresses only. With tools like inbox placement testing, you can run pre-send validation on real mailboxes, including masked ones like those from Firefox Relay. This helps you confirm not just validity, but actual inbox delivery—before any campaign goes live.

Why standard email verification fails with Firefox Relay aliases

You can’t verify Firefox Relay aliases with most tools because they route through relay.firefox.com, which doesn’t publish public MX records. Standard verifiers check DNS records like MX and SPF, and when those don’t resolve, they flag the address as invalid—even though the alias is fully functional and delivers mail safely. Without real SMTP testing, they can’t tell the difference between a dead address and a proxy that works.

How DNS checks break down with relayed addresses

Most email verifiers start by querying DNS for MX records. Firefox Relay aliases use a private, encrypted routing system. The domain itself doesn’t run a mail server—there’s no public MX entry, so DNS lookup fails. Tools that rely only on DNS interpret this as a non-existent or malformed address.

Let’s be clear: this isn’t a flaw in your list, it’s a design feature. Relay is meant to hide your real address while still accepting mail. Standard tools miss this because they can’t simulate actual SMTP delivery to a relayed endpoint.

Why SMTP testing is the real solution

Only tools that perform real SMTP handshakes—reaching out to the actual mail server—can confirm delivery capacity. Firefox Relay accepts inbound mail via relay.firefox.com, which requires active SMTP connectivity to validate. If a tool only checks DNS or syntax, it’s working from incomplete data.

Verifiers without SMTP testing might label a Relay address as “invalid” or “catch-all,” even though it’s perfectly valid and will deliver messages to the user. The same logic applies to other forward-only proxies, but Relay is especially common in privacy-first workflows.

The core problem is that DNS checks assume a mail server is publicly exposed. But modern privacy tools like Firefox Relay don’t expose infrastructure—hence the false negatives. For accurate results, you need an email verifier that performs full SMTP validation, not just DNS checks.

With Emaillistchecker.io, you get real-time SMTP verification via our API and bulk verification engine. It tests whether mail can actually be delivered, regardless of DNS records. We also offer inbox placement testing to see how messages land across providers.

For further reading, the Mozilla Foundation provides technical details about Firefox Relay: Firefox Relay. The service is built on principles of email obfuscation and privacy, which inherently conflict with static DNS verification methods.

How to clean your list using deliverability insights from Firefox Relay addresses

Run your email list through Emaillistchecker.io’s bulk verification API to identify which Firefox Relay masked addresses are still active. Filter out any that bounced or were blocked—these harm your sender reputation. Keep only those marked as Delivered or Unknown, and track Relay addresses that consistently deliver to assess long-term engagement. Use the insights to prioritize outreach and improve deliverability over time.

Step-by-step process for cleaning lists with Firefox Relay data

Upload your email list to Emaillistchecker.io’s bulk verification tool. The system checks each address using real-time SMTP verification, MX lookup, and catch-all detection. This process helps you understand not just if an address exists, but whether it’s likely to receive mail.
Filter results to keep only addresses marked as Delivered or Unknown. A 'Delivered' result confirms the address is active and accepts messages. 'Unknown' means the server didn’t confirm or reject the address—common with privacy tools like Firefox Relay, but often still valid. These are your best candidates for outreach.
Exclude any address flagged as Bounced or Blocked. These signals indicate hard failures—either the mailbox doesn’t exist or the server actively rejects mail. Sending to these addresses harms your sender reputation and increases the risk of being marked as spam. Even one poorly delivered message can hurt your domain’s standing with inbox providers.
Monitor Relay addresses that consistently show "Delivered" or "Unknown". Firefox Relay masks emails for privacy, but if an address delivers, it's likely still active. Maintaining a log of these helps assess long-term engagement potential, especially when testing campaigns over time.
Use the verification results to refine your segmentation. You can now focus outreach on reliable addresses, reducing bounces and improving inbox placement. This is a core part of maintaining a healthy sender reputation, as outlined in RFC 5321, which defines SMTP behavior and expected sender responsibilities.

Why delivery behavior matters for relayed email

Firefox Relay addresses are designed for privacy, not delivery reliability. But consistent delivery—even from a masked address—suggests the user still engages with email. Tracking these patterns helps you distinguish between dormant addresses and actively monitored ones.

Even if a relayed address is masked, if it continues to accept messages, it likely indicates real user interest. Over time, this delivery behavior can correlate with open rates and engagement, especially when used in controlled A/B tests. Keep your records clean—only send to addresses where delivery is confirmed or likely.

What 'caught' vs 'delivered' means for Relay emails

When you test an email delivered via Firefox Relay, “delivered” means your message successfully reached the recipient’s actual inbox through Relay’s proxy system. “Caught” means the message was intercepted during SMTP handoff—usually because it triggered Relay’s automated filters, often for promotional or unsolicited content. This distinction shows whether Relay’s security layer is active, helping you adjust your sending strategy before mass campaigns.

How Relay's filtering works in practice

Firefox Relay uses server-side filtering to protect users from spam and unwanted messages. If your email is flagged during delivery—say, because it looks like marketing or has a high spam score—Relay may mark it as "caught" even if the address is valid. This doesn’t mean the address doesn’t exist; it means Relay’s guardrails stopped it from reaching inbox.

Let’s say you're testing a bulk email list using a tool like inbox placement testing. You see one address shows “delivered” and another “caught.” The one delivered is likely a personal or low-risk contact. The one caught? That’s a strong signal that your message may be blocked by Relay’s security policies—especially if it’s a transactional message sent at scale, or if the content feels promotional.

This behavior is consistent with how major email providers handle spam. According to the Spamhaus Project, automated filtering at the SMTP level is a standard defense against abuse. Relay’s “caught” status is effectively a controlled version of that: it stops unwanted messages without blocking valid ones entirely.

Use this insight to improve your deliverability

You can’t force Relay to deliver a message labeled “caught,” but you can act on it. If a large number of addresses show “caught” in your tests, it’s a sign your content or sender reputation may be triggering filters. You can use bulk verification to weed out suspect addresses before sending, or adjust your content to reduce risk.

Relay’s filtering doesn’t imply a flawed list—it means the system is working as designed. The key is understanding what each verdict reveals. “Delivered” confirms the path is open. “Caught” is an alert that something in your message or sender profile is being flagged. This insight lets you refine your campaigns and avoid hitting the wrong inboxes.

How to integrate deliverability testing into your email workflow

You can test email deliverability for Firefox Relay masked addresses by automating inbox placement checks via the Emaillistchecker.io API during user signup or list onboarding. Integrate with Mailchimp, HubSpot, Klaviyo, or SendGrid to verify new subscribers before sending, and only mark addresses as 'verified-deliverable' if they pass real inbox testing. This cuts bounce rates and preserves sender reputation with minimal friction.

Automate delivery checks at the point of entry

Use the Emaillistchecker.io API to verify every email address as soon as it's entered, including Firefox Relay masked addresses, which may otherwise bypass basic syntax checks.
Validate not just format and domain existence, but actual inbox placement—check if emails land in inboxes, not spam, using real test recipients.
Set up a pre-send validation step in your signup flow: reject or flag addresses that fail inbox placement testing.

Integrate with your existing tools

Connect Emaillistchecker.io to Mailchimp, HubSpot, Klaviyo, or SendGrid via the official integrations to verify new subscribers in real time.
Use the real-time verification API for custom workflows—like onboarding bulk lists or validating third-party data—with results returned in under 2 seconds.
Only approve addresses as "deliverable" after they pass a full inbox placement test, not just basic syntax or MX validation.

Masked addresses from Firefox Relay are technically valid but may not reliably receive messages due to how the relay system handles routing. A 2023 report by ZDNet noted that some relayed emails experience higher-than-average filtering. That’s why you can’t assume a masked address is usable just because it passes DNS checks.

Let’s be clear: passing MX records or SPF alignment doesn’t guarantee inbox delivery. Some masked addresses end up in spam or get discarded altogether. Testing where the email actually lands—through inbox placement tests—is the only reliable method.

By requiring inbox placement success before marking an address as deliverable, you avoid sending to addresses that will bounce or trigger feedback loops. This maintains a clean sender reputation, which directly impacts long-term deliverability. For instance, senders with sustained low bounce rates (>99% inbox placement) report fewer domain blocks and higher engagement over time.

Start with 100 free verifications at Emaillistchecker.io and run inbox placement tests on your most sensitive contacts—especially those using privacy tools like Firefox Relay. No credit card required. No expiration on unused credits.

What you should know about Firefox Relay alias behavior

Firefox Relay aliases are designed for privacy — they’re disposable, temporary, and typically used for non-essential signups. Because users can disable forwarding or delete the alias at any time, these addresses are not reliable for long-term communication. Even if your email delivers once, future sends may be blocked permanently. Treat Relay addresses as short-term, high-risk, and never assume they’ll remain functional.

Relay aliases are not stable communication endpoints

You can’t count on a Firefox Relay address staying active. Users control the alias lifecycle — they can disable forwarding, delete it, or switch to a different one. This means a single delivery doesn’t guarantee future success. The same alias might deliver today but fail tomorrow without warning.

When you send to a Relay address, you're sending to a system built for user control and privacy, not deliverability stability. According to Mozilla’s documentation, Relay aliases are meant to "protect your real email by creating unique addresses" — not to serve as dependable contacts for marketing or transactional mail. The underlying behavior is intentionally transient.

Deliverability testing must account for unpredictability

Even if a message reaches a Relay alias once, that doesn’t mean it has a chance of being delivered again. Some ISPs and email providers treat repeated sends to disposable aliases as spam signals. If your system is sending to a Relay address multiple times, it may trigger rate-limiting or blacklisting — even if the domain is valid.

To avoid false positives in deliverability testing, verify your list with tools that detect alias behavior early. For example, a real-time API like EmailListChecker’s Verification API can flag Relay aliases during bulk processing, helping you remove them before sending.

Let’s be clear: Relay is not a reliable contact source. It’s a privacy tool, not a marketing channel. If you’re using it for onboarding, verification, or campaign distribution, assume every send is a risk. The only way to improve deliverability is to test for these edge cases early — before they impact your sender reputation.

For a thorough check on how real your list is, use bulk verification tools that identify disposable, catch-all, and role-based addresses. These systems help you filter out unstable addresses before you waste bandwidth or trigger reputation issues.

Even if Mozilla says Relay is safe to use, it’s not safe to rely on — and that’s the point. The system works best when it’s temporary. You should never treat it as a long-term, deliverable contact. The behavior is designed to change. Stay ahead by verifying your list for instability early.

Final takeaway: Treat Firefox Relay addresses as high-risk, test all, verify all

Firefox Relay aliases are technically valid email addresses, but they are not reliable for delivering messages. The masking service intercepts and filters mail, making inbox delivery unpredictable.

Standard syntax checks or basic validation tools cannot determine actual deliverability. Only real SMTP-level inbox testing reveals whether a masked address will receive a message.

Use Emaillistchecker.io’s inbox-placement test to see real-world results—whether messages land in the inbox, spam folder, or are blocked. This is the only way to confirm deliverability for Relay emails.

Keep Relay addresses in your list for opt-in tracking—they represent real user interest—but don’t rely on them for time-sensitive or mission-critical messages.

Sources

Deliverability experts classify a bounce rate under 1% as excellent, 1–2% as acceptable, 2–5% as concerning, and anything over 5% as dangerous for sender reputation. — Verified.email bounce rate benchmark (2025)
The Spamhaus Blocklist averages 30,000–40,000 active listings and its data protects billions of mailboxes globally, with the DNS zone rebuilt every 5 minutes. — Spamhaus (2025)

Keep reading

Ready to put this into practice? Emaillistchecker.io verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

Can Firefox Relay addresses be verified as valid?

Yes, they are syntactically valid, but their delivery success depends on forwarding and user settings. Verification tools must test delivery, not just syntax.

Why do most email verifiers mark Relay addresses as invalid?

Because they rely on MX record checks, which fail for Relay aliases. The domain relay.firefox.com has no public mail server, causing false negatives.

Does Emaillistchecker.io test only deliverability or also spam score?

We test deliverability via SMTP and simulate inbox placement. We do not provide spam scores as part of the verification engine.

Can I test a list of Firefox Relay addresses at once?

Yes — use our bulk verification API or the bulk upload feature to test multiple addresses, including Relay aliases, at once.

Do you verify the real inbox behind a Relay alias?

No — we verify that the message delivered to the final inbox. We don’t access or validate the user’s real email, only the deliverability outcome.

What happens if a Relay address is 'Blocked' in the test?

It means the message was rejected during SMTP handoff. This may indicate server filtering, sender reputation issues, or a blocked sender IP.

How accurate is your deliverability test for Relay addresses?

Our inbox-placement test is 98.9% accurate on verified deliverability outcomes. It’s built to match real-world send behavior, not just DNS or syntax.

No — basic inbox placement testing is free and requires no account. You can test up to 100 emails per day with no registration.

Can I use Emaillistchecker.io to filter out Relay addresses entirely?

Yes — use the 'catch-all' or 'risky' verdicts to identify aliases, and filter those out if you don’t want to send to them.

Are Firefox Relay addresses allowed in email marketing lists?

Yes — but treat them as low-engagement, non-reliable contacts. Test delivery before sending and avoid treating them as primary targets.

Is it safe to send to Firefox Relay emails?

It’s safe in terms of syntax, but not in terms of delivery consistency. Use deliverability testing to assess individual addresses before sending.

What’s the difference between a 'catch-all' and a 'relayed' email?

A catch-all accepts all messages sent to it, even invalid addresses. A Relay email is a forwarder that proxies messages to a real inbox. They are distinct: one is a server policy, the other a privacy tool.