Here is the output from dovecot -n
# 2.2.9: /etc/dovecot/dovecot.conf # OS: Linux 3.13.0-32-generic x86_64 Ubuntu 14.04.2 LTS ext4 auth_mechanisms = plain login auth_verbose = yes log_path = /var/log/dovecot.log mail_debug = yes mail_location = maildir:/var/mail/vhosts/%d/%n namespace inbox { inbox = yes location = mailbox Drafts { special_use = \Drafts } mailbox Junk { special_use = \Junk } mailbox Sent { special_use = \Sent } mailbox "Sent Messages" { special_use = \Sent } mailbox Trash { special_use = \Trash } prefix = } passdb { args = /etc/dovecot/dovecot-sql.conf.ext driver = sql } plugin { sieve = ~/.dovecot.sieve sieve_dir = ~/sieve } protocols = lmtp imap pop3 sieve service auth-worker { user = vmail } service auth { unix_listener /var/spool/postfix/private/auth { group = postfix mode = 0666 user = postfix } unix_listener /var/spool/postfix/private/dovecot-auth { group = postfix mode = 0660 user = postfix } unix_listener auth-userdb { mode = 0600 user = vmail } user = dovecot } service imap-login { inet_listener imap { port = 0 } } service lmtp { unix_listener /var/spool/postfix/private/lmtp { group = postfix mode = 0600 user = postfix } } service pop3-login { inet_listener pop3 { port = 0 } } ssl_cert = </etc/dovecot/dovecot.pem ssl_cipher_list = ALL:!LOW:!SSLv2:ALL:!aNULL:!ADH:!eNULL:!EXP:RC4+RS A:+HIGH:+MEDIUM ssl_key = </etc/dovecot/private/dovecot.pem userdb { args = uid=vmail gid=vmail home=/var/mail/vhosts/%d/%n driver = static } doveconf: Error: protocols: Unknown protocol: sieve protocol lda { deliver_log_format = msgid=%m: %$ mail_plugins = sieve postmaster_address = postmaster quota_full_tempfail = yes rejection_reason = Your message to <%t> was automatically rejected:%n%r } protocol imap { imap_client_workarounds = delay-newmail mail_max_userip_connections = 10 } protocol pop3 { mail_max_userip_connections = 10 pop3_client_workarounds = outlook-no-nuls oe-ns-eoh } doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf: protocols: Unknown protocol: sieve
PS dovecot-sieve is installe. Also this error appears without sieve in the list of protocols in /etc/dovecot/dovecot.conf