what SHOULD be a simple postfix issue

[Bill Earl]

I've done this before.  I've done this MANY times before.   The times
I've done it before I had no problems doing it.  But today, I can't
figure this out, so any help would be appreciated.  :-)

On a test machine, postfix is sending out mail showing that it's coming
from addresses like username at machinename.domainname.com  but I'd like it
to send the mail with from addresses like username at domainname.com    I
cannot find what to change in main.cf to make that happen!   Yes, I'm
typing "postfix reload" to make my changes take effect.  ;-)

Can anyone jumpstart my brain?   Thanks much.  :-/

Bill