Error Messages When Sending Emails

[JetJock]

I have various email accounts set-up in MS Outlook.

Everytime I send an email to anyone with a Hotmail,
Yahoo, Vodafone, ATTGlobal. AOL address (and others), I
get a systems administrator message stating that the
adress is not a gateway!

This has only just started to happen!

Any suggestions or solutions?

Thanks