When an SMTP server performs an action, it will give a three
digit code which tells exactly what went on. The most common codes are
2XX 4XX and 5XX. The first number of the code is:



2 - The SMTP server has accepted the command, and has completed the request

4 - There was a temporary failure with the server or command. Try again later.

5 - There was a permanent error, and the requested operation will never be completed.



Some example codes:



250 - Everything was completed/accepted without errors

450 - “requested mail action not taken”  - Generally this is given out when there is no such user on the server

550 - The email cannot be sent, either because of and administrative
prohibition on the host/email address that the mail is coming from, or
because relaying is not allowed