Authentication Error with Dovecot

[Kevin Laurie]

Hello,
I have some email authentication error with dovecot.
Could someone tell me what is the problem that is indicated below?
Is it due to some plugin?


Apr 18 21:56:35 auth-worker(2057): Info: mysql(127.0.0.1): Connected
to database vmail
Apr 18 21:56:35 imap-login: Info: Login: user=<user at mydomain.net>,
method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=2058, secured,
session=<n/0fGAUUgwB/AAAB>
Apr 18 21:56:35 imap: Error: Can't load plugin imap_acl_plugin: Plugin
acl must be loaded also (you must set: mail_plugins=$mail_plugins acl)
Apr 18 21:56:35 imap: Fatal: Couldn't load required plugins
Apr 18 21:56:53 imap-login: Info: Disconnected (auth failed, 1
attempts in 2 secs): user=<user2>, method=PLAIN, rip=127.0.0.1,
lip=127.0.0.1, secured, session=<z2gKGQUUiQB/AAAB>
Apr 18 21:57:05 imap-login: Info: Login: user=<user2 at mydomain.net>,
method=PLAIN, rip=127.0.0.1, lip=127.0.0.1, mpid=2065, secured,
session=<5tmoGQUUjQB/AAAB>
Apr 18 21:57:05 imap: Error: Can't load plugin imap_acl_plugin: Plugin
acl must be loaded also (you must set: mail_plugins=$mail_plugins acl)
Apr 18 21:57:05 imap: Fatal: Couldn't load required plugins