Ho do I masquerade the "user@user.local" address in mail/mailx?

Hi,

I'm brand new here and looking for a solution:

I'm using mail or mailx. The default reply address is �myshortusername@mylongusername.local� which makes absolutely no sense for anybody receiving my emails.

But how do I change it? There seem to be many solutions but none for Mac OS X. I'm using 10.5.

Where are the files that I need to change? What do I need to add or change?

Thanks a lot for your help!

Gary