Free reference

SMTP & Bounce Error Library

Every SMTP and bounce code, explained in plain English. One page per code, organized by what's actually breaking.

Hard Bounce

Permanent delivery failures indicating the recipient address is invalid or does not exist.

Deferral

Temporary server-level issues where the message is queued for retry.

Policy

Rejections due to security policies, spam filters, or sender blocklists.

Authentication

Failures caused by missing or invalid SPF, DKIM, or DMARC records.

Mailbox

Issues specific to the recipient's mailbox, such as being full or disabled.

Content

Rejections based on message size, attachments, or media types.

Rate Limit

Temporary blocks due to sending too many emails too quickly.

Connection

Network, routing, or SMTP protocol errors during transmission.

Concept

General email deliverability concepts and terminology.