You don't need to delete your old SSL. Basically, this article will solve your first problem ("MAILER-DAEMON@server.example.com" )
https://docs.cyberpanel.net/doku.php?id=ssl-for-mailserver
It will set the domain as mail server hostname, here you need to choose the domain which is also set as RDNS. I would suggest you to first create a website such as mail.yourdomain.com, then issue mail server SSL.
Then also set this domain as RDNS, and the second article is to help you solve your 587 issue.
I have done this, sir. However, I am still using GoDaddy’s SSL, and because of this, I cannot login my mail via 587 port. Because my own SSL does not support mail services. If I can remove my SSL, and then I can associate let’s encrypt ssl, it will work :s