RFC 6186 provides SRV records format to help service discovery:
_submission._tcp SRV 0 1 587 mail.nasqueron.org. _imap._tcp SRV 0 1 143 mail.nasqueron.org. _pop3._tcp SRV 10 1 110 mail.nasqueron.org.
IMAP is preferred to POP3 with a 0 against 10 priority, lower = better. Mail user agents will then use IMAP if supported.