1.0 Error:

While configuring the Dovecot Mail Delivery Agent, the Dovecot server would not start and the following error messages were printed.

$ sudo systemctl start dovecot
Job for dovecot.service failed because the control process exited with error code. See "systemctl status dovecot.service" and "journalctl -xe" for details.
$ systemctl status dovecot.service
● dovecot.service - Dovecot IMAP/POP3 email server
   Loaded: loaded (/lib/systemd/system/dovecot.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2018-02-05 04:25:20 UTC; 39s ago
     Docs: man:dovecot(1)
           http://wiki2.dovecot.org/
  Process: 3983 ExecStart=/usr/sbin/dovecot (code=exited, status=89)
$ sudo journalctl -xe
...
Feb 05 04:25:20 Example systemd[1]: Starting Dovecot IMAP/POP3 email server...
-- Subject: Unit dovecot.service has begun start-up
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit dovecot.service has begun starting up.
Feb 05 04:25:20 Example dovecot[3983]: doveconf: Warning: NOTE: You can get a new clean config file with: doveconf -n > dovecot-new.conf
Feb 05 04:25:20 Example dovecot[3983]: doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: protocols: Unknown protocol: si
Feb 05 04:25:20 Example systemd[1]: dovecot.service: Control process exited, code=exited status=89
Feb 05 04:25:20 Example systemd[1]: Failed to start Dovecot IMAP/POP3 email server. 
-- Subject: Unit dovecot.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit dovecot.service has failed.
-- 
-- The result is failed.
Feb 05 04:25:20 Example systemd[1]: dovecot.service: Unit entered failed state.
Feb 05 04:25:20 Example systemd[1]: dovecot.service: Failed with result 'exit-code'.

The syslog had following messages.

...
Feb  5 04:25:20 Example dovecot[3983]: doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: protocols: Unknown protocol: sieve
Feb  5 04:25:20 Example systemd[1]: dovecot.service: Control process exited, code=exited status=89
Feb  5 04:25:20 Example systemd[1]: Failed to start Dovecot IMAP/POP3 email server. 
Feb  5 04:25:20 Example systemd[1]: dovecot.service: Unit entered failed state.
Feb  5 04:25:20 Example systemd[1]: dovecot.service: Failed with result 'exit-code'.

2.0 Solution

The problem was resolved by installing the dovecot-sieve and dovecot-managesieved packages.

$ sudo apt-get install dovecot-sieve dovecot-managesieved