What is a well-known service that binds port 25 and is it required on all hosts?

What is a well-known service that binds port 25 and is it required on all hosts?

A.
SNMP and it should be turned off if not needed.

B.
SMTP and it is a required service.

C.
SMTP and it is only required on MX hosts.

D.
SLPD and it is required if you run LDAP services.

E.
SSHD and it is required for secure logins.

Explanation/Reference:
From /etc/services:
smtp        25/tcp        mail

smtp is needed only on mail exchange servers.