The Default Port list is meant, I believe, to allow Mail to negotiate a working server at various locations, if you were to be moving about with a laptop. At a single venue, such as home, there is no reason not to establish the port manually as we instructed. They did not tell you Port 25 would not be working (although likely it will soon not be), but "suggested", it appears, that you establish the port to be 587. Thus it is best to take the options away, and simply override the choice of port.
If Port 25 were made non-working, then Mail would likely negotiate to present the outgoing server on Port 587 of its own accord.
The Default Port did not appear in Mail until 10.5.5 and Mail 3.5, as best I remember (I could check, but little need to).
Still not sure what the tech was telling you about the Incoming Server.
Ernie