When you start mail service it’s configured to deliver mail on the local network.
When mail service is enabled, it’s set up as follows:
SMTP, POP, and IMAP are active and use standard ports.
Junk mail and virus filters are on.
Blacklist and greylist filters are off.
Quotas aren’t enforced.
Standard authentication methods are used (not Kerberos), with POP and IMAP set for Digest (CRAM-MD5), and Digest-MD5, and SMTP authentication turned off. If your server is an Open Directory master, Kerberos, Digest (CRAM-MD5), and Digest-MD5 are used.
Mail is delivered only locally (no mail is sent over the Internet).
Mail relay is unrestricted.