I haven't used any free Windows mail servers in ages, sorry.

Okay, so you have [email protected], who's currently using mail.example.com as his POP3 and SMTP mail server. You want him to use the new internal server instead.

If the new mail server is called internal.example.com, and tom's account on it is then [email protected], you need to configure his email client like this:
  • POP3 Server: internal.example.com
  • SMTP Server: internal.example.com
  • Name: Tom
  • Email: [email protected]
  • Username: tom
  • Password: ********
Emails will then appear to come from [email protected], even though they're going through the internal server.

The only place you need to use [email protected] is on the ISP-end mail server when you configure the forwarding inwards.