Where can I find a list of SMTP error codes?

SMTP error codes are documented in several authoritative sources:

Core standards:

RFC 5321: Basic SMTP protocol and reply codes (2xx, 3xx, 4xx, 5xx). RFC 3463: Enhanced Mail System Status Codes (X.Y.Z format). RFC 3464: Delivery Status Notifications format.

ESP documentation:

SendGrid, Mailchimp, Postmark, and others publish bounce code guides specific to their platforms.

Provider-specific references:

Google: Gmail postmaster help documentation. Microsoft: Exchange and Microsoft 365 error code references. Yahoo: Postmaster guidelines.

Third-party resources:

Deliverability blogs often maintain comprehensive code lists. Email tool vendors publish interpretation guides.

Start with the RFCs for the standard. Then consult provider-specific documentation for their custom variations.