This is commonly used as a strategy for spam prevention. It's stating that mail relaying is not allowed for some reason. It may be due to the email address or authentication settings used.
How to fix SMTP Error 551
Ensure proper email authentication.
Allow SMTP relays (if you are certain you want that.)
email/troubleshooting/error-numbers/551.txt · Last modified: 2015/02/04 14:47 by dlevey
