One of the most common errors to come across in web hosting is a 550 error. This error can have many reasons and causes to why it is shown; however, the error itself always indicates one thing: that there is a mail relay error.
Common 550 Error Messages
Mail bounces across several receptors as it sent from sender to recipient, allowing several places where an error can occur should a relay point be out and have an error message generated. Although there can be a variety of messages generated, HostGator has listed some of the most common messages to accompany a 550 error.
550 No Such User Here
This error occurs when sending to a remote location while having a DNS zone on the server for the domain set to localdomains or automatically checked.
550 DICTIONARY ATTACK
This error indicated that all email for that account must be stopped from being sent and pause all mailing lists. Then wait one hour. If Outlook was being used, users can try using webmail to send messages.
550 External MTA's must be authenticated in order to send e-mails
Outlook needs the Outgoing Server Requires Authentication option checked.
550 SMTP Error -or- 550 Sender verify failed
"Perhaps you have not logged into the POP/IMAP server in the last 30 minutes or do not have SMTP Authentication turned on in your email client."