sudden email error on one domain


Secret Agent

I'm suddenly getting this error in Outlook for my domain that has bene working fine all along.

Task User - Sending' reported error (0x800CCC78) : 'Unable to send the message. Please verify the e-mail address in your account properties. The server responded: 550 5.1.8 invalid/host-not-in-DNS return address not allowed'

The site is fine, I receive fine but smtp seems to be the trouble suddenly. I am not sure why. Anyone have an idea about this?

The site is up, dns is fine too.