Sendmail: Domain of sender address does not exist

[Jeroen Lankheet]

Hi guys,

Just installed a new server with FC3. I kept my old mail config files. I
have put them in their place and mail (spam ) started coming in. I have one
problem that I cannot send emails through my own mail server. 
The error is "Domain of sender address user at mydomain.com does not exist". I
have a lot of hits when searching on this error but none give me proper
handles to check and fix things. 

Thanks for the help.
Jeroen