Email Sender (no replies)

Hi All

I have the weirdest thing with WebDev. I am getting the following error message when trying to send an email:

Content of Email.Sender not recognised by server. Transaction denied.

It is weird because this exact same sender can send emails from other programs to this smtp server. And this web app can send emails to smtp servers on other servers. Any ideas?

Nowhere else to look at this point.

Cheers
Andre