Dovecot upgrade from 2.3 to 2.4

•  see link for detail guideline, Archlinux provide dovecot23 as alternative
• add two new configuration, see link
• dovecot_config_version = 2.4.2
• dovecot_storage_version = 2.4.2
• delete empy dict

#dict {
#quota = mysql:/etc/dovecot/dovecot-dict-sql.conf.ext
#expire = sqlite:/etc/dovecot/dovecot-dict-sql.conf.ext
#}

• change "disable_plaintext_auth = yes" to "auth_allow_cleartext = no", see link
• rename userdb and passdb. 
• passdb authenticated the user. 
• userdb lookup then retrieves post-login information specific to the authenticated user

userdb passwd {
          #driver = passwd
}
passdb pam {
          #driver = pam
          passdb_driver=pam
}

• update mail location, see example on link.

#mail_location = mbox:~/mail:INBOX=/var/mail/%u
#mail_location = mbox:~/mail:INBOX=/var/mail/%{user}

mail_driver = mbox
mail_path = ~/mail
mail_inbox_path = /var/mail/%{user}

• update ssl, see linklink.

#ssl_cert = </etc/letsencrypt/live/hongy19.net/fullchain.pem
#ssl_key =  </etc/letsencrypt/live/hongy19.net/privkey.pem
#ssl_dh =   </etc/dovecot/dh.pem
ssl_server_cert_file = /etc/letsencrypt/live/hongy19.net/fullchain.pem
ssl_server_key_file = /etc/letsencrypt/live/hongy19.net/privkey.pem
ssl_server_dh_file = /etc/dovecot/dh.pem

• rename "service_count = 1" to "service_restart_request_count = 1", see link.