This error indicates an inability to send emails.
There are many reasons for a 550 error.
550 DICTIONARY ATTACK
You must stop sending emails and pause your mailing lists. Then wait one hour. If you were using Outlook, during the wait you 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 550-server in the last 30 minutes or do not have SMTP Authentication turned on 550 in your email client."
