error-handling

Engineering guides: frameworks, pipelines and data imports

Greylisted and Mailbox Busy Responses: Why Verification Returns Unknown

Discover why email verification returns 'unknown' for greylisted or busy mailboxes. Learn how Emaillistchecker.io reduces false unknowns with 98.9%.

Engineering guides: frameworks, pipelines and data imports

402 Payment Required: Handling Insufficient Credits Errors in 2026

Fix the 402 payment required error when your API runs out of credits. Learn how to detect, prevent, and recover from insufficient credits in email.

Engineering guides: frameworks, pipelines and data imports

401 Unauthorized Email Verification API Key Troubleshooting in 2025

Fix 401 unauthorized errors in email verification APIs with real steps. Reduce failed requests, fix authentication, and keep your list hygiene strong.

Engineering guides: frameworks, pipelines and data imports

Fixing Email Verification API 400 Bad Request Invalid Email Format Error

Resolve the 400 Bad Request Invalid Email Format error in your email verification API integration.

Engineering guides: frameworks, pipelines and data imports

Retry Policy for Email Verification: Which Errors Are Safe to Retry?

Learn which email verification errors are safe to retry and how to implement idempotent retry rules to reduce false bounces and improve list hygiene.

Engineering guides: frameworks, pipelines and data imports

What to Do When Email Verification API Returns Unknown Verdict

Learn what to do when your email verification API returns an unknown verdict. Understand the cause, reduce false positives, and improve deliverability with prov