587 is one of the 3 default ports Mac Mail lists, the other 2 are 25 & 465.
The error I get is:
Cannot send message using the server mail.mydomain.com.
The server “mail.mydomain.com” refused to allow a connection on the default ports.
Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be delivered.
The message has my actual domain name instead of "mydomain", I just changed that to protect the innocent
When it checks my email, I get a certificate error, but I am given the opportunity to bypass that by clicking "Allow".