Ecco la spiegazione dal supporto tecnico del mio fornitore:
We think you should turn off [automatically manage connection
settings]; there should never be any reason for it to use different
ones. You always want it to stick with the recommended port 587
(etc.), no matter what.
The trouble with that feature is that if your ISP connection stops
working for a second, and Apple Mail can't connect, it will think "oh,
the settings might be wrong, so let's try a different port". It tries
something else at random, and maybe it works, because your ISP
connection is working again. But Apple Mail wrongly assumes the port
setting was the trouble -- so it saves the new setting! And then
you're using something weird that only works on certain networks,
etc., when it would have been better to keep the original "works
everywhere" port number.
Apple Mail is the only program we know of that does anything like
this, and it's not a good idea, really; it's designed to help people
if their mail server company changes things, but can cause problems in
other cases. We've even seen cases where it randomly changes the
settings to something that can never possibly work on any servers,
although it's not supposed to.
A riprova di questa spiegazione, quando l'ho spento e ho rivelato le impostazioni della porta, erano diverse per ogni account (ma tutte erano sbagliate).