Comment by stonesweep

This makes no sense (too abstract of a statement), but here let's try this:

    $ man systemd | head -4 | tail -1
       systemd, init - systemd system and service manager

An email daemon (Postfix, Exim, Dovecot, UW-IMAP, Sendmail, etc.) are services running on a server. An "email server" would be a server running a daemon to provide email service in this example.