How do I change the "mail from" for email alerts?

Harold J. Ballinger wrote:

The exim list introduced me to a "qualify_domain" setting and I was able to force it that way. Thanks so much for your help.
  

It's great that you finally got it working - but in retrospect, setting
the from address is really the job of the mail application (MUA) which
the MTA should simply pass along if it's sane.

For future reference, mailx and postfix work "out of the box" for this
sort of thing - no special hacks, MTA tweaks or secret settings required.

Joe