The mail delivery protocol status codes report failures involving the message delivery protocol. These failures include the full range of problems resulting from implementation errors or an unreliable connection.
- 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