What are Gmail “reason codes”?

Gmail reason codes identify specific rejection causes:

Common reason codes in enhanced status:

5.7.1: Generic policy rejection (spam, blocked). 5.7.26: Authentication requirement not met. 5.7.27: SPF record problem. 5.7.28: DKIM signing problem. 5.1.1: User does not exist.

How they appear:

"550-5.7.26 This message does not have authentication information." Code embedded in multi-line response. Often with explanatory text and support link.

Using reason codes:

Map to specific issues. Guide remediation efforts. Track patterns over time. Build automated classification.

Documentation:

Google publishes error code references. Postmaster Tools provides additional context.

Gmail reason codes are diagnostic precision. They tell you exactly which check failed.