I’ve tried going to Webmin Configuration > Sending Email, checking “Enable SSL” and setting “Use port number” to 587, but when I click save, it automatically unchecks “Enable SSL” and automatically set “Use port number” to default port. It cant be changed!
You’re misunderstanding what that log entry means. It doesn’t matter what port Usermin uses. It runs locally…it can reach the local postfix on any available port. This is a problem for outgoing mail, and one you can’t overcome for every receiving mail server on the internet. You’ll need to use a mail relay as @calport suggested.
Calport, I am not sure why it is that one would want to let me know…i stopped using Google Cloud a long time ago because i spent 2 years with a mail server running through a relay host due to this exact problem with them!
I think i may have actually been one of the first ones on this forum to experience that, and Google cloud i think were one of the first service providers to restrict ALL mail on standard ports (not just 25…they have blocked them all including SSL ports for mail). The only way around this with Google Cloud it to either use a relay, or use GSuite where some ports are allowed via the internal network.
One of the relay hosts that i have looked at and used an0nimo are Sendgrid. There are others.