Incoming email does not work: Fatal Plugin '$mail_plugins' not found from directory /usr/lib/dovecot/modules

Symptoms

• Incoming email does not arrive

• Mail log /var/log/maillog contains messages similar to:

  Oct 20 17:36:53 plesk dovecot: lda(35096): Fatal: Plugin '$mail_plugins' not found from directory /usr/lib/dovecot/modules
  Oct 20 17:36:53 plesk dovecot: lda(35099): Fatal: Plugin '$mail_plugins' not found from directory /usr/lib/dovecot/modules 
  Oct 20 17:36:53 plesk dovecot: lda(35101): Fatal: Plugin '$mail_plugins' not found from directory /usr/lib/dovecot/modules 
  Oct 20 17:36:53 plesk dovecot: lda(35103): Fatal: Plugin '$mail_plugins' not found from directory /usr/lib/dovecot/modules

Cause

Due to Dovecot syntax changes in version 2.4, the file /etc/dovecot/conf.d/15-lda.conf is no longer required.

When Postfix uses Dovecot LDA for local delivery, it calls dovecot-lda as the delivery agent.
If 15-lda.conf was misconfigured (e.g., referencing $mail_plugins incorrectly), Dovecot LDA would fail, causing Postfix to defer local deliveries.

https://doc.dovecot.org/main/installation/upgrade/2.3-to-2.4.html#plugin-settings

Resolution

1. Connect to the server via SSH

2. Rename /etc/dovecot/conf.d/15-lda.conf

   # mv /etc/dovecot/conf.d/15-lda.conf /etc/dovecot/conf.d/15-lda.conf_bk

3. Restart Dovecot:

   # systemctl restart dovecot