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Migrating from Courier-IMAP to Dovecot on Linux

Dovecot imap logo

Due to recurring performance and service issues, I recently migrated the IMAP service of a mail server from Courier-IMAP to Dovecot. The latter is known to be more robust and actively maintained. In this article, I will provide the notes that helped me accomplish this migration. They are specific to my architecture (Courier-IMAP + Courier-SMTP with authuserdb authentication), but I hope they can help others as well. Of course, taking a backup is highly recommended before making any modifications. Good reading!


  • Install Dovecot:
root@host:~# apt update && apt install dovecot-imapd

Dovecot provides the script, which allows migrating from Courier-IMAP while preserving IMAP UIDs by converting the courierimapuiddb file to a dovecot-uidlist file. Source:

  • Download and add execute permission:
root@host:~# wget root@host:~# chmod +x



  • Run converting tool:
root@host:~# ./ --to-dovecot --recursive --convert /data/vmail/

Users accounts

  • For each user, we need to add Dovecot password. To do so, we can use the doveadm pw command and add the username and password to the /etc/dovecot/passwd file:
root@host:~# doveadm pw -s ssha512 -p <PASSWORD>
  • Example here with the password:
root@host:~# doveadm pw -s ssha512 -p {SSHA512}8lhtRxyXWP5azW1OjllkVUSuMPVABNtTR/MxMVTIEYXLXjoOMwCe/7Bpr1iqPi/nKutRbPxvBddG1pk2BVpgorhZPjI=
  • Edit the /etc/dovecot/passwd file with username:{SSHA512}sha512generatedpassword:{SSHA512}8lhtRxyXWP5azW1OjllkVUSuMPVABNtTR/MxMVTIEYXLXjoOMwCe/7Bpr1iqPi/nKutRbPxvBddG1pk2BVpgorhZPjI=
  • Edit the /etc/dovecot/conf.d/auth-passwdfile.conf.ext file:
passdb { #authentication method driver = passwd-file #specifies the format of the username in the password file. args = username_format=%u /etc/dovecot/passwd } userdb { driver = static #specifies the home directory path for the mailbox, where %d represents the domain and %n represents the username. args = uid=vmail gid=vmail home=/data/vmail/%d/%n }


  • Edit the /etc/dovecot/conf.d/10-ssl.conf file:
ssl = yes #courier certificates: #ssl_cert = <etc/courier/imapd.pem #Note : split imapd.pem from -----BEGIN PRIVATE KEY----- to the end to create server.key #ssl_key = <etc/courier/server.key #Let's encrypt certificates: ssl_cert = <etc/courier/fullchain.pem ssl_key = <etc/courier/privkey.pem
  • Edit the /etc/dovecot/conf.d/10-mail.conf file:
mail_location = maildir:/data/vmail/%d/%n/Maildir namespace { prefix = INBOX. separator = . inbox = yes }
  • Edit the /etc/dovecot/conf.d/10-auth.conf file:
auth_mechanisms = plain !include auth-passwdfile.conf.ext
  • Restart the dovecot service:
root@host:~# systemctl restart dovecot.service

Correction of errors after migration

After the installation and migration, I had to make some modifications, which I will detail here.

The folders are appearing duplicated in Thunderbird

  • Disable the /etc/dovecot/conf.d/15-mailboxes.conf configuration file:
root@host:~# mv /etc/dovecot/conf.d/{15-mailboxes.conf,15-mailboxes.conf.disabled} root@host:~# systemctl restart dovecot.service

Maximum number of connections

Complete error: imap-login: Maximum number of connections from user+IP exceeded (mail_max_userip_connections=10): user=<> :

  • Edit the /etc/dovecot/conf.d/20-imap.conf file:
mail_max_userip_connections = 30 root@host:~# systemctl restart dovecot.service

Process_limit reached

Complete error: master: Warning: service(imap-login): process_limit (100) reached, client connections are being dropped

  • Edit the /etc/dovecot/conf.d/10-master.conf file:
default_process_limit = 200 root@host:~# systemctl restart dovecot.service

Inotify instance limit for user

Complete error: Warning: Inotify instance limit for user 7200 (UID vmail) exceeded, disabling. Increase /proc/sys/fs/inotify/max_user_instances

  • Edit the /etc/sysctl.conf file:
fs.inotify.max_user_instances = 65535 fs.inotify.max_user_watches = 65535 root@host:~# sysctl -p /etc/sysctl.conf root@host:~# systemctl restart dovecot.service

Out of memory

Complete error: imap(<1123794><2epGbh79Cui4hVBj> Fatal: master: service(imap): child 1313798 returned error 83 (Out of mem ory (service imap { vsz_limit=256 MB }, you may need to increase it) - set CORE_OUTOFMEM=1 environment to get core dump)

  • Edit the /etc/dovecot/conf.d/10-master.conf file:
default_vsz_limit = 512M root@host:~# systemctl restart dovecot.service
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