there is a note
either the note is left over from a previous circumstance or it is not RFC compliantIf start-tls='''tls-only''', port 465 will be used
http://www.iana.org/assignments/service ... umbers.txt
https://tools.ietf.org/html/rfc8314#section-7.3
https://www.mailgun.com/blog/which-smtp ... 5-465-587/
https://sendgrid.com/blog/whats-the-dif ... 5-and-587/
effectively making RouterOS a legacy system, which should default to 587.
If Mikrotik would like to deviate from this default then such separate setting should be implemented, it is up to the client (RouterOS) to refuse using a non-established TLS connection on port 587 if tls-only is checked.
When a "/tool e-mail set port" is not defined and "/tool e-mail send" is attempted there is no communication even attempted to the email server defined for "/tool e-mail send server", however an error message of "error connecting to server (6)" is still given, so Mikrotik doesn't seem to default to any port, when any "/tool e-mail set port" is defined it works but then port mentioned in the note for start-tls="tls-only" is overridden by the value in "/tool e-mail get port").
In addition to this, the Send Email tool (/tool e-mail send) in Winbox has only a TLS checkbox (not pulldown with no, tls-only, yes) which doesn't indicate which configuration is attempted (implies tls-only). Especially as if /tool e-mail set start-tls=''no" and your "/tool e-mail send" TLS is checked it will still send the email without TLS.