A number of plugins are also available to monitor mail servers. The mail server itself (Mail Transport Agent (MTA)) is monitored by
check_smtp, and the mail queue on the mail server can be checked with
check_mailq. Since the latter test takes place locally, the plugin is described in the next chapter in 7.8 Regularly Checking the Status of the Mail Queue (page 180).
To monitor the Mail User Agent (MUA) protocols POP3 and IMAP —including the SSL variants, POP3S and IMAPS—the plugin
check_tcp is used.
check_pop and so forth are symbolic links to
check_tcp, which determines which protocol it should test by means of the name by which it is called, and makes the relevant presettings.