A mail user agent is for reading, replying, disposing, and composing an email. If you move /usr/lib/sendmail to /usr/lib/oldsendmail on a Solaris system and insure it's not running on the server (ps -ef|grep mail), then when you go to send an email with mailx you will get an error (even on a local user within the same system).

# mailx -s"test" unixops < /etc/issue
/usr/lib/sendmail: No such file or directory


Sendmail does not have to be running (that is when it is allowing incoming mail to the local server) but must be present (or some other MTA) for sending email.

On HP-UX, moving sendmail does nothing to sending to local users but will stop you from sending out from that server to another.