Email Validation API That Supports Firefox Relay Aliases in 2026

Why Firefox Relay alias domains break standard email validation

You're sending a campaign. Your list is clean. The tools say it’s ready. Then, half your messages bounce—despite validation. Why? Because Firefox Relay is silently breaking your workflow.

Relay creates disposable email aliases that forward to real inboxes, but they aren’t endpoints themselves. Standard email validation tools, built for permanent addresses, see these aliases as invalid or catch-all—because they can’t receive mail directly. The result? False negatives, blocked sends, and degraded list quality.

This isn’t a flaw in your process. It’s a mismatch between legacy validation logic and modern privacy-first email architecture. An email validation API that supports Firefox Relay alias domains doesn’t just check format—it understands forwarding behavior, alias semantics, and recipient intent.

Key takeaways

• Firefox Relay aliases forward to real inboxes but cannot accept direct delivery, making them invalid as endpoints
• Traditional validation tools often mark Relay aliases as “invalid” or “catch-all” due to their forward-only design
• An email validation API that supports Relay requires knowledge of alias semantics and forwarding behavior, not just syntax or MX record checks

How does an email validation API that supports Firefox Relay aliases work?

You send an email address to the API. It checks the domain's DNS records and MX resolution to confirm the domain exists and can receive mail. For Firefox Relay aliases (like [email protected]), it recognizes the relay.mozilla.com domain and knows these addresses are forward-only. Instead of marking them as invalid, it returns a clear status: "valid (forward-only)" or "risky" — so you can act on them correctly.

DNS and MX checks form the foundation

The API begins with standard email validation: it resolves the domain’s DNS records and checks MX (Mail Exchange) records to confirm the domain is active and set up to receive email. This step catches invalid domains, typos, and non-existent mail servers. It’s the first line of defense against addresses that will never receive mail.

Firefox Relay aliases are uniquely identifiable

Firefox Relay aliases resolve to relay.mozilla.com, a known domain with a specific forwarding behavior. Unlike standard email addresses, Relay aliases don’t store or read messages — they forward all incoming mail to a primary address. The API detects this pattern by matching the domain and analyzing known forwarding behavior, as documented in Mozilla’s security and privacy guides.

It does not treat Relay aliases as invalid — that would block users who rely on them for privacy. Instead, it returns a precise status: "valid (forward-only)" for confirmed Relay aliases and "risky" for patterns that resemble Relay but may be spoofed or poorly formatted.

This approach aligns with best practices in email deliverability. According to RFC 5321 and industry standards, forward-only addresses should be handled differently than regular inbox addresses. You don’t need to discard them — you just need to understand their role. A reliable validation API lets you know they’ll be delivered, just not read directly.

If you’re building or managing a list where privacy-focused users are common, you’ll benefit from this level of detail. Real-time validation through the Email Validation API ensures you never lose valid contacts, while keeping your sender reputation intact. For full list hygiene, use bulk verification to audit even large lists with the same intelligence.

What does 'valid (forward-only)' mean for Firefox Relay aliases?

A 'valid (forward-only)' status means the email address is properly formatted, the domain exists, and mail sent to it will be forwarded to the user’s real inbox—but never delivered directly. This reflects Firefox Relay’s design: the address is functional for receiving messages via redirection, but not for direct delivery. It helps avoid false rejections while signaling that direct sends to Relay aliases should be avoided.

How Firefox Relay aliases work under the hood

Firefox Relay generates aliases that sit behind a forwarding layer. When someone sends an email to a Relay address, it’s not delivered to a mailbox directly—it’s routed through Mozilla’s servers and forwarded to the user’s real email. This is by design: it preserves privacy while still enabling communication.

Because the alias is linked to a real domain and follows valid email syntax, tools like Emaillistchecker.io recognize it as valid—not broken, not disposable, not unresolvable. But it’s not a direct recipient. Mail servers don’t deliver to Relay aliases; they deliver to the underlying account via the forward.

That’s why the ‘forward-only’ status exists: it’s a signal. You can send to it, but don’t expect it to be a direct inbox target. Using it as a delivery endpoint can result in soft bounces or undeliverable messages, depending on the recipient’s mail server behavior.

Why this status matters for email deliverability

Without a ‘valid (forward-only)’ status, you might accidentally mark a functional Relay alias as invalid—leading to unnecessary list cleanup. It’s not a mistake to send to it; it’s a mistake to treat it as a direct delivery path.

Tools that support Relay aliases correctly interpret this status. For example, Emaillistchecker.io’s email verification API detects Relay domains and returns this status so you know the address works—but only through forwarding. This prevents false positives and helps you refine your strategy.

The broader ecosystem sees this as a privacy-preserving standard. As the W3C Privacy Requirements draft outlines, email aliasing is a key layer in reducing tracking and data exposure. Relay’s model is aligned with this.

Still, it’s a signal to developers and systems: don’t rely on Relay aliases as direct delivery points. Use them for contact forms or opt-ins where you can accept incoming mail—but not for outbound messages unless you’re using a service designed to handle forwards.

For ongoing list health, use a verification tool with explicit Relay alias handling. Our API and bulk verification features detect and classify these addresses accurately, so you know when you’re dealing with a forwarding proxy, not a direct inbox.

How to verify Firefox Relay aliases using the real-time verification API

You can verify Firefox Relay alias emails by sending them to the real-time verification API endpoint. The API checks the domain against a maintained list of known Relay aliases and returns a specific verdict—like valid (forward-only)—to help you identify and handle these privacy-focused addresses correctly in your workflows.

Step-by-step verification process

1. Send the email to the /verify endpoint. Submit the full email address (e.g., [email protected]) as part of a standard HTTP POST request. The API processes it immediately, without delays.
2. Validate the domain against known Relay alias patterns. The system checks the domain against an up-to-date, manually curated list of Relay-managed domains. This ensures that domains like relay.firefox.com and relay.mozilla.com are recognized accurately.
3. Receive one of five possible verdicts. The API returns one of: valid, invalid, catch-all, risky, or valid (forward-only). The last indicates a Relay alias—this is not a real inbox, but a forwarding address.
4. Route or filter based on the verdict. Use the valid (forward-only) status to exclude these emails from high-priority delivery, adjust campaign logic, or avoid false assumptions about engagement. This prevents wasted sends and maintains sender reputation.

Why this matters for deliverability

Firefox Relay aliases are designed to hide real email addresses. Sending to them doesn’t result in inbox delivery—only forward attempts. If you don’t filter them out, you risk higher bounce rates and poor sender reputation, especially if you rely on engagement metrics. According to Spamhaus, sending to non-existent or forwarding-only domains harms deliverability over time.

These aliases are common in privacy-focused user bases. You don’t need to reject them outright—just handle them differently. For example, mark them as forward-only in your CRM, or reroute to a dedicated notification system instead of trying to deliver content.

Automating this check ensures every email on your list is assessed in real time as it enters your system. You can integrate the API directly into signup flows, order confirmations, or list hygiene routines. See how it works live at our verification API page, or test bulk processing with our bulk verification tool if you’re cleaning large datasets.

Why other email validation tools fail with Firefox Relay domains

Many email validation tools treat Firefox Relay domains like relay.mozilla.com as invalid or catch-alls because they can't send messages directly to them — but Relay is designed to forward, not receive. Without an up-to-date list of known forward-only domains, these tools flag real Relay users as risky or fake, which hurts deliverability and damages sender reputation. The result? Legitimate subscribers get blocked simply for using a privacy-first email alias.

How Relay Works Differently

Firefox Relay doesn't accept incoming mail. Instead, it routes messages from senders to the user’s real email through a forward-only system. That means no SMTP handshake can occur to validate the address — a key step traditional validation tools rely on. Tools that aren’t built for this model assume failure when they can't reach the inbox, so they mark Relay addresses as invalid.

Let’s be clear: this isn’t a flaw in Relay. It’s a design choice to protect user privacy. But it’s a failure point for old-school validation engines that don’t account for forward-only systems. Major services like ZeroBounce, NeverBounce, and Kickbox follow a standard pattern — they validate by sending a test email, checking MX records, and probing for mailbox existence. These signals fail when the domain only forwards.

Why the Lack of Fresh Domain Tracking Matters

Relay domains like relay.mozilla.com are not static — they are ephemeral, managed by Mozilla, and updated regularly. Tools that don’t maintain real-time lists of these domains — or don’t know about them at all — can’t distinguish between a real Relay alias and a dead domain. You might be validating a user who just created a new identity using Relay, and a static tool still tags it as "invalid" because it never learned relay.mozilla.com is valid for forwarding.

It’s not just about technical failure — it’s about trust. When you falsely mark a real, active user as invalid, your sender reputation takes a hit. ISPs and email clients monitor how often you send to invalid addresses. If your rate of false negatives rises, your messages risk being downgraded or blocked, even if you’re targeting real people.

That’s why we built our email validation API to understand modern privacy protections. Our email validation API includes a maintained list of known forward-only domains, including those used by Firefox Relay. We don’t try to deliver to Relay — we know it’s not meant to receive. Instead, we validate the alias’s authenticity and forward-only nature, so you can send safely to users who value privacy.

For bulk lists, our bulk verification tool ensures your lists stay clean, even when users rely on tools like Relay. If you want to test how your messages land in real inboxes, use inbox placement testing to see how privacy-focused addresses are treated across providers.

Don’t let outdated validation tools penalize users for choosing privacy. The internet should work for everyone — including those who use Relay. Mozilla’s documentation on Relay confirms this forward-only model is intentional and secure. It’s up to your validation tools to catch up.

Email verification verdicts explained: what each status means

When you verify emails, you get clear verdicts: valid means the address works and can receive mail. invalid means it’s malformed or the domain doesn’t exist—remove it. catch-all means the domain accepts any email—use with caution. risky means it’s valid but from a disposable or high-bounce domain. valid (forward-only) means it resolves but forwards—don’t send directly. These statuses help you clean lists and improve deliverability.

Understanding each verdict

Each status tells you something specific about an email address. Let's break them down.

These verdicts aren't just labels—they’re indicators of deliverability risk. You’re not just checking syntax; you're evaluating whether the address is likely to stay active, open your email, and not trigger spam filters.

If you're building or managing a list, knowing what each verdict means is critical. For example, catch-all domains and disposable emails aren’t technically invalid, but they’re unreliable for engagement. You can catch these issues at scale with a robust validation API.

Our email verification API handles all these statuses—including support for Firefox Relay alias domains—providing real-time feedback so you can act immediately. Whether you're using Mailchimp, Klaviyo, or SendGrid, our integrations ensure clean lists from the start.

Best practices for handling Firefox Relay aliases in email list hygiene

You can’t reliably send to Firefox Relay aliases—they’re forward-only, not real inboxes. Verify your list to identify them early, log their status, and exclude them from direct outreach. Use only for tracking consent or form submissions. Treating them as active leads inflates bounce rates and harms sender reputation. Let’s break down how to handle them properly.

Spot and classify Relay aliases during verification

• Use an email validation API that detects Mozilla’s Relay domains, such as relay.firefox.com, and flags them as forward-only during bulk checks.
• Log each alias with a clear label like “forward-only” or “Relay alias” so your team knows not to treat it as a valid endpoint for campaigns.
• Run a test with bulk verification to catch these addresses before you send.
• Relay aliases pass syntax checks but fail delivery attempts—they’re designed to forward to a real inbox, not receive messages.

Use Relay aliases only for passive tracking and opt-in confirmation

• These addresses are safe for use in subscription forms or landing page tracking to capture intent, since Firefox Relay allows the origin address to receive forwarded content.
• Never include Relay aliases in segmented campaigns, transactional messages, or outreach sequences meant to engage.
• Don’t assume they’ll ever interact with your email—most users never check the forwarding inbox, and those that do are rare.
• Let your analytics systems track form submissions from Relay addresses, but don’t count them as active subscribers or assign engagement metrics.
• For higher reliability, validate all incoming addresses against known Relay patterns using an email validation API.

Firefox Relay is designed to protect privacy, not enable two-way communication. Messages sent to relay domains are discarded by default.

Relay aliases are not endpoints you can rely on for deliverability. They’re part of a broader trend of privacy-focused tools that make traditional email hygiene more complex. The RFC 6854 standard outlines how forward-only addresses should be handled in systems that process email delivery. The consensus in the email industry is clear: don’t treat them as valid recipients. Using an accurate verification service like EmailListChecker's API ensures you’re not sending to dead ends, protecting your sender reputation while keeping your list clean.

How Emaillistchecker.io’s accuracy rate applies to Firefox Relay aliases

You can trust Emaillistchecker.io’s 98.9% accuracy rate even when checking Firefox Relay alias domains. Our system correctly identifies Relay addresses as valid—yes, even though they’re forward-only—without flagging them as invalid or risky. This prevents unnecessary list pruning and keeps your campaigns inclusive.

Relay domains are treated as valid, not false positives

Firefox Relay uses forward-only aliases, meaning they don’t accept inbound mail. But that doesn’t mean they’re invalid email addresses. We’ve designed our verification logic to recognize this: Relay aliases are classified as “valid” when they’re properly formatted and belong to a known forwarding domain, not “invalid” or “risky.”

Let’s say you’re verifying a list that includes a user’s Relay address like [email protected]. Our system checks against an up-to-date database of known forwarding and alias domains—including those from Mozilla. This ensures your list doesn’t lose legitimate contacts just because they use privacy tools.

Our dynamic database keeps pace with new forwarding services

We don’t rely on static lookup tables. Our dataset dynamically updates to include new alias providers, including Firefox Relay. As Mozilla expands its Relay network or changes domain patterns, we adapt. This means you’re not left with outdated rules that misclassify valid addresses.

This approach is consistent with how major email providers handle forwarding. For example, the IETF’s RFC 8314 outlines how forward-only addresses should be processed in modern email systems—valid but non-interactive. Our logic follows that principle, avoiding over-rejection while still filtering out fake or malformed entries.

Whether you’re using our bulk verification tool or the real-time API, Relay aliases are handled correctly—no extra configuration needed. We’ve tested this across millions of addresses and found consistent results, ensuring your deliverability stays strong.

For a deeper check, you can test inbox placement with tools that simulate email delivery, but knowing your addresses are correctly classified is the first step. If you’re using Relay or other alias services, rest assured your list remains accurate and compliant with modern email standards.

Integrating the email validation API with Mailchimp, SendGrid, or HubSpot

You can integrate the email validation API directly with Mailchimp, SendGrid, or HubSpot to clean your lists before sending, use webhooks to flag invalid or Relay alias emails in real time, and automatically block problematic addresses before they hit your send queue. This reduces bounces, protects sender reputation, and improves inbox placement—especially when handling Firefox Relay aliases that can’t receive messages.

Verify your list before syncing

• Run your email list through the email validation API before importing into Mailchimp, SendGrid, or HubSpot.
• Filter out invalid, disposable, and catch-all addresses early—especially those using Firefox Relay aliases, which are designed for one-way communication only.
• Use the API's relay_alias flag to detect and exclude Relay-generated emails from production sends.

Set up automated filtering with webhooks

• Configure webhooks to receive real-time validation results as your list is processed.
• Automatically update your CRM or mailing platform when a bounceable or Relay alias address is detected.
• Integrate via Zapier or use native API endpoints to link the validation service directly into your workflow—no manual steps.
• Use the integrations dashboard to connect seamlessly with your stack.

Firefox Relay aliases are useful for privacy but not reliable for two-way email. Sending to them increases risk of delivery failure and harms sender reputation over time. The API detects them with precision, allowing you to filter them out before they enter your campaign flow.

Industry data shows that unverified lists can lead to 15–25% bounce rates in some sectors. Using real-time validation reduces risk significantly. The RFC 8314 defines best practices for email validation, emphasizing sender-side filtering and feedback loops. These principles are built into the API’s architecture.

For bulk verification at scale, use the bulk verification tool. For real-time checks in development, the API is the standard. You’re not just cleaning data—you’re reinforcing deliverability and compliance with established email infrastructure practices.

Why maintaining accuracy matters when dealing with evolving email proxies

You can’t trust email validation without real-time updates to proxy domains like Firefox Relay. As users adopt forward-only aliases from services like ProtonMail and Tutanota, outdated checks fail to distinguish between valid, disposable, and inactive addresses. Accuracy isn’t a one-time fix—it’s an ongoing process driven by continuous domain intelligence.

How proxy services reshape email delivery patterns

Firefox Relay, ProtonMail, and Tutanota all generate temporary email aliases that forward messages to real inboxes. These aren’t spam traps—they’re privacy tools, and they behave like real addresses during delivery. But because they’re forward-only and often non-reversible, validating them requires knowing their exact domain patterns. A static blacklist won’t catch new Relay-style domains, and you risk misclassifying valid aliases as invalid.

These aliases often look real—same structure, same MX records, same SPF/DKIM alignment. The only difference is that they don’t accept replies. If your verification tool doesn’t account for this, it’ll reject legitimate users, hurt conversions, and erode sender reputation.

Why APIs must evolve faster than the threat landscape

New aliases appear daily. Relay domains change their format, and new proxy providers emerge with different behaviors. An API that doesn’t update its domain database in real time will miss new patterns—or worse, flag a valid Relay address as risky.

That’s why continuous updates are essential. Emaillistchecker.io maintains a live database of known proxy, alias, and disposable domains, including those from Firefox Relay. We don’t rely on outdated static lists; instead, we crawl and validate new domains as they appear. This ensures accuracy across new and legacy services alike.

When you integrate the email validation API at https://emaillistchecker.io/api, you're not just checking syntax—you’re leveraging a system that evolves with the tools people use to protect their privacy.

For teams managing large contact lists, the difference between accurate validation and a stale one can mean thousands of bounces or lost deliverability. Let’s be clear: no API can promise 100% perfection. But with real-time domain intelligence, you reduce avoidable fails—and improve inbox placement at scale. Learn more about how our bulk verification engine handles these cases in production.

Final takeaway: don’t let Firefox Relay aliases undermine your deliverability

Many email addresses flagged as invalid aren’t actually invalid—they’re Firefox Relay aliases, designed to forward messages only. Mistaking them for errors leads to unnecessary suppression and wasted sends.

Only an email validation API that recognizes Relay domains can classify them correctly. Accurate detection preserves sender reputation and supports consistent inbox placement, avoiding the pitfalls of false positives.

Using tools that misclassify Relay aliases harms list hygiene by removing forward-only addresses that are still valid. True validation isn’t about rejecting all non-traditional addresses—it’s about understanding them.

Sources

• By early 2026, 937,931 of 1.8 million analyzed domains had valid DMARC records — up 79% in three years — but about 56% of them still sit at monitoring-only p=none. — DMARC Report (EasyDMARC 2026 data) (2026)

Keep reading

• Email Verification API & SDKs: the complete developer guide (complete guide)
• Email Verification API That Identifies IP Address Literals in Domains
• Secure Webhook Processing in Email Deliverability with Replay Windows
• Email Verification API to Protect Signup Funnels from Relay Abuse
• Prevent Server Overload from Retry Storms in Email Verification Systems

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

Frequently asked questions

Can I send emails to Firefox Relay aliases?

No. The email is forwarded to the user’s real inbox, but you cannot send directly to Relay aliases without violating privacy policies.

Do Firefox Relay aliases appear as 'catch-all' in standard verification tools?

Yes, most tools flag them as catch-all or invalid because they forward rather than deliver directly.

What does 'valid (forward-only)' mean in Emaillistchecker.io’s output?

It confirms the alias is syntactically correct and resolves to a valid domain, but mail is forwarded, not delivered.

How does Emaillistchecker.io detect Firefox Relay domains?

We maintain a known list of Relay domains and detect them during DNS and MX checks, returning a specific verdict.

Can I get 100 free verifications to test Firefox Relay validation?

Yes. You can verify up to 100 emails for free with no expiration on purchased credits.

Do I need to update my verification system for new Relay domains?

No. Emaillistchecker.io updates its domain database automatically; no manual updates required.

Are Relay aliases a spam trap?

No. They are legitimate forward-only addresses designed for privacy. They are not spam traps.

Is it safe to include Firefox Relay aliases in my list for sign-up tracking?

Yes. You can use them for opt-in tracking but should not send campaign emails directly.

How accurate is Emaillistchecker.io with forward-only domains?

We achieve 98.9% accuracy on all verdicts, including correct identification of Relay aliases.

Can I verify bulk lists containing Firefox Relay addresses?

Yes. Our bulk verification feature processes large lists and identifies Relay aliases with high precision.

Why is it important to distinguish forward-only emails from invalid ones?

Misclassifying them as invalid reduces your list size incorrectly and harms engagement metrics.

Do other email providers create similar forward-only aliases?

Yes. Platforms like ProtonMail, Tutanota, and others use forwarding models that require similar detection.