questions about SSL for my mail server

for https on my webserver I used Let's Encrypt for mydomain.net & www.mydomain.net, on another machine I just finished setting up and testing my postfix dovecot sasl spamassassin mailserver, I've been looking into getting an SSL certificate for my mail server, do I get a trusted certificate for mail.mydomain.net for use with postfix and dovecot ? my new email address is myuserid@mydomain.net, currently with my automated self signed certificates that were created whan I installed the mail servers, when I do a mail security test it tests positive for encrypted emails, I don't have a trusted Certificate "unknown authority", & hostname mismatch.