Common Server Error Codes
This article explains common mail server error codes.
Remote host said: 550 sorry, no mailbox here by that name
Our mail server tried to deliver the message but the other server said that no email address by that name exists. This usually means that the email address is misspelled.
Remote host said: 554 Transaction Failed Spam Message not queued
Our server tried to deliver the message but the other server flagged it as spam and rejected it.
553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
Make sure that "my server requires authentication when sending mail" is enabled in your mail client.
firstname.lastname@example.org Mail quota exceeded.
The users email box is full and cannot accept any other incoming email.