Permanent Failures (5xx)
The email cannot be delivered and should not be retried. Remove these addresses from your list.
Temporary Failures (4xx)
The email may be delivered if retried later. Implement proper retry logic with backoff.
Policy Rejections
The email was rejected due to policy. Check authentication and sender reputation.
Permanent Failures
550 5.1.1Permanent FailureUser Unknown / Mailbox Not Found
The email address does not exist on the receiving server.
550 5.1.2Permanent FailureBad Destination Mailbox Address
The destination mailbox address is invalid or malformed.
550 5.2.1Permanent FailureMailbox Disabled
The recipient's mailbox has been disabled or suspended.
550 5.4.1Permanent FailureRecipient Address Rejected
The recipient server rejected the address for relay.
553 5.1.3Permanent FailureInvalid Address Format
The email address syntax is invalid.
550 5.5.1Permanent FailureCommand Not Recognized
The SMTP command was not recognized by the server.
Temporary Failures
421 4.7.0Temporary FailureConnection Rate Limited
Too many connections from your IP address.
452 4.2.2Temporary FailureMailbox Full / Over Quota
The recipient's mailbox has exceeded its storage quota.
451 4.3.0Temporary FailureMail System Full
The receiving mail system is temporarily unavailable.
450 4.1.1Temporary FailureRequested Action Not Taken
Temporary failure - mailbox unavailable.
Policy Rejections
Prevent Email Errors Before They Happen
Whylidate validates email addresses before you send, helping you avoid bounces and protect your sender reputation.