For scale and redundancy reasons we operate each service from its own hostname. So we always recommend using the most relevent hostname for incoming and outgoing servers in mail clients. These are as follows:
IMAP: imap.stackmail.com (or imap.domain)
POP: pop.stackmail.com (or pop.domain)
SMTP: smtp.stackmail.com (or smtp.domain)
However, for compatibility reasons and to help ease migrations, we automatically setup mail.domain for each of your packages.