Dovecot-2.4 introduced breaking change to its entire configuration system and
Dovecot-2.3 configuration files will NOT work after upgrade. Please read
https://doc.dovecot.org/2.4.1/installation/upgrade/2.3-to-2.4.html
before upgrading. We strongly recommend finalizing your Dovecot 2.4 configuration on a test system before upgrading any production systems.
The following steps typically make the upgrade process easier:
1. Make a note of your current configuration by running doveconf -nP 2. Stop the dovecot daemon 3. Move ALL your configuration files to a temporary location 4. Upgrade to dovecot-2.4.x 5. Read the new configuration files and uncomment as necessary 6. Compare the new doveconf -n output with the old one and add missing configuration settings one by one while checking that the system works after each change
https://doc.dovecot.org/2.4.1/installation/upgrade/2.3-to-2.4.html
before upgrading. We strongly recommend finalizing your Dovecot 2.4 configuration on a test system before upgrading any production systems.
The following steps typically make the upgrade process easier:
1. Make a note of your current configuration by running doveconf -nP 2. Stop the dovecot daemon 3. Move ALL your configuration files to a temporary location 4. Upgrade to dovecot-2.4.x 5. Read the new configuration files and uncomment as necessary 6. Compare the new doveconf -n output with the old one and add missing configuration settings one by one while checking that the system works after each change