This message looks an answer of the remote SMTP server. You should search for the outgoing SMTP session in the delivery logs, to check which remote SMTP server is answerd this, and double check if it's the correct MX for the domain you are trying to send to.

Maybe a DNS cache issue and you got the wrong mx entry ?

Hi Greg,

That error is a remote error and not a smartermail error.

Ideally the remote mail administrator needs to resolve that issue.