x.5.xxx Mail Delivery Protocol Status

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