Your question probably really belongs over in
Mac OS X 10.6 Snow Leopard -> Mail & Address Book where you're likely to get a more targeted response than what I'll offer..
Mail.app presumes that SMTP servers will be using standard SMTP ports during the configuration. If Mail
absolutely hangs when trying to establish the connection to the SMTP server, there are a few work-arounds. I say "
absolutely hangs" because I have seen Mail go SPOD for several minutes while trying to establish connections to SMTP servers..
The quickest workaround is to assign another SMTP server from an existing account temporarily when setting up the account, and then add and assign the proper SMTP server to the account after setup is completed. You can add and change SMTP servers for accounts in Mail->Preferences->Accounts->{account name}->Account Information. At the bottom of that tab is "Outgoing Mail Server (SMTP)" popup list; select "Edit SMTP Server List" from that menu and you can add a new SMTP server there and assign a custom SMTP port to the new server.
-Douggo