I'm currently making the move from Sendmail to Postfix. My current (Sendmail) mailserver delivers most incoming mail to local mailboxes, but some of the incoming mail needs to be forwarded to another external mailserver (mail.externaldomain.dom) which is listening to a non standard port (10025).

In Sendmail, this involved creating a mailer that delivers to port 10025 ("ESMTP10025"), then create a rule in mailertable.db that specifies all mail addressed to mail.externaldomain.dom should use the new mailer, then simply forward the incoming mail to mail.domain.dom in virtusertable.

So for instance, mail arrives at my mailserver address to user1@mydomain.dom. Virtusertable contains a line that reads:

Sendmail then uses the ESMTP10025 mailer to forward the incoming mail to port 10025 on mail.externaldomain.dom.

This works perfectly in Sendmail.

But how do I do this with Postfix?

Have you tried

relayhost = mail.externaldomain.dom:10025

or some derivative for virtual relay/transport?

Have a look at this while you're about it

http://www.postfix.org/transport.5.html

Yes, I had ssen the relayhost directive, but doesn't that forward all mail to the specified host?

Cool! That seems to be exactly what I need! Thanks!

As I said, you may need one of the virtual transport map versions depending on exactly what you are trying to do. What mail will go to this server/port?