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RFC 1893 - Enhanced Mail System Status Codes
RFC 3463 - Enhanced Mail System Status Codes
RFC 2142 - Mailbox Names for Common Services, Roles and Functions
Full Mail Box Bounces
Novell GroupWise - Email rejected with 500 5.5.0 Syntax error - line too long
WindowSecurity.com - Understanding E-mail Spoofing

RFC 3463

RFC 3463           Enhanced Mail System Status Codes        January 2003

         2.X.X   Success
         4.X.X   Persistent Transient Failure
         5.X.X   Permanent Failure

Appendix A - Collected Status Codes

         X.1.0     Other address status
         X.1.1     Bad destination mailbox address
         X.1.2     Bad destination system address
         X.1.3     Bad destination mailbox address syntax
         X.1.4     Destination mailbox address ambiguous
         X.1.5     Destination mailbox address valid
         X.1.6     Mailbox has moved
         X.1.7     Bad sender's mailbox address syntax
         X.1.8     Bad sender's system address

         X.2.0     Other or undefined mailbox status
         X.2.1     Mailbox disabled, not accepting messages
         X.2.2     Mailbox full
         X.2.3     Message length exceeds administrative limit.
         X.2.4     Mailing list expansion problem

         X.3.0     Other or undefined mail system status
         X.3.1     Mail system full
         X.3.2     System not accepting network messages
         X.3.3     System not capable of selected features
         X.3.4     Message too big for system

         X.4.0     Other or undefined network or routing status
         X.4.1     No answer from host
         X.4.2     Bad connection
         X.4.3     Routing server failure
         X.4.4     Unable to route
         X.4.5     Network congestion
         X.4.6     Routing loop detected
         X.4.7     Delivery time expired

         X.5.0     Other or undefined protocol status
         X.5.1     Invalid command
         X.5.2     Syntax error
         X.5.3     Too many recipients
         X.5.4     Invalid command arguments
         X.5.5     Wrong protocol version

         X.6.0     Other or undefined media error
         X.6.1     Media not supported
         X.6.2     Conversion required and prohibited
         X.6.3     Conversion required but not supported
         X.6.4     Conversion with loss performed
         X.6.5     Conversion failed

Bron: RFC 3463 - Enhanced Mail System Status Codes