Q: Mail keeps changing my SMTP server settings

We have to use port 25 for our outgoing mail in order to make our ISP and mail provider play nice. It was working fine for 6+ months, and in the last month or so it's been mysteriously changing the SMTP server to 587. We go into our settings, change it back to 25 (NOT default, specifically 25), it works for a while, and then one day it's not sending again. So we check the smtp settings, and lo and behold, it's back to 587.

We've changed ISPs for other reasons, and the problem has persisted. We've worked with our mail provider and pretty much eliminated them. So now we're left with a freakish Mail hiccup that we can't solve (and is a potentially big problem because we're a small business, but not everyone who uses the computer has any clue how to change smtp settings).

We'd like to stay with Mac mail because everyone knows how to use it, so we're hoping to get some help from the forums before we give up and move over to Thunderbird.