[ILUG] changing domain when sending external e-mail

[Tony Patton]

OK here is the situation:
  my mail server is xxx2.nwifhe.net
  other mail server xxx1.nwifhe.ac.uk

  xxx1 is used to send mail externally,
  xxx2 is our internal-only mail server.

How can I make sure that mail that is going externally ends with
nwifhe.ac.uk rather than xxx2.nwifhe.net, but remains untouched for mail
that is staying internally?

here is what I get when sending a message thru sqwebmail:
  <ilug at linux.ie>: host mail.tuatha.org[194.125.145.45] said: 501
    <pattont at mis.nwifhe.net>... Sender domain must exist

All help appreciated
Tony Patton