For scale and redundancy reasons we operate each service from its own hostname. So we always recommend using the most relevant hostname for incoming and outgoing servers in mail clients. These are as follows:
IMAP: imap.yourdomain.com
SMTP: smtp.yourdomain.com
However, for compatibility reasons and to help ease migrations, we automatically setup mail.domain for each of your packages.
Any clients already using these hostnames can continue to do so when the account is moved to controlpanel.Systems, and any new accounts added at controlpanel.Systems will have the same functionality.