550 5.2.1 [SDNF] Sender Domain Not Found

The "550 5.2.1 [SDNF] Sender Domain Not Found" error message indicates that our server was unable to verify the existence of a sender's email domain - in other words, the part of the email address after the "@" doesn't seem to exist.

It's possible that this problem is temporary, in which case you should try sending your message later. If you get this error repeatedly, you should check with your DNS provider to ensure that the domain is resolvable, or use a tool such as MXToolbox to verify the status of the domain.