Do anyone else have the problem that for some reason at random times, Apple Mail seems to decide to CHANGE the SMTP outgoing port #.
Our ISP requires port 587 to be used, but every few weeks (days), Apple Mail changes it to port 25… causing us to have to reconfigure the SMTP port settings yet again…
Sometimes it helps to find the config file and make it read only.
I assume you can read the logs and see that something fails and than Apple Mail auto corrects your port.
This may help some users.
After you set the port the way you want, turn off the check box on “automatically manage connection settings”. It’s in preferences under account settings where you change the port numbers. There is a check box for both incoming and outgoing I believe.
I’ve seen this happen a few times myself with different people. I think it’s more common when you change networks often. If mail is open and fails during a change it might try a different port. If the server actually has multiple port options it can end up making a bad choice.