E-mail servers are configured to match your domain for an additional convenience. This applies to POP3/IMAP and SMTP address of the servers.
Example: If your domain name is domain.com then the addresses of incoming and outgoing e-mail servers will be:
POP3 server: mail.domain.com on port 110 (default)
IMAP server: mail.domain.com on port 143 (default)
SMTP server: mail.domain.com on port 25 (default) or port 26