Have recently encountered this problem with setting up a new discourse instance on digital ocean server, support says that ports 587, 465, and 25 aren’t supported but 2525 should work. Set SMTP setting to port 2525 but that still isn’t working to send activation e-mail with Brevo, may be some other problem.
Looks like SendGrid may be a better option and/or better supported with that server provider, d.o. support wrote:
“You can also use REST API with SendGrid, which allows you to send emails via HTTP requests instead of the traditional SMTP protocol if it is suitable for you or make use of another third-party sending service.”
Don’t know how REST API works with HTTP, apparently that may be better than SMTP?