Sl mail server host names problem

My sl mail server has been running without fault for sometime. Just recently I found emails sent by the server can't be replied too.

Sending and receiving are both fine, but if someone replies there reply fails. For some reason the server is adding its hostname to the outbound email address.

Should be me@apple.com.au but its now sending as me@osxserver.apple.com.au

Thus the reply fails with an error.

What's the best way to resolve this? My dns looks ok, as does the mail config.

Thanx

Julian