Creating Roundcube filters on Plesk server with Courier-IMAP fails: Unable to connect to Sieve server

Symptoms

• Roundcube is used as webmail on a Plesk for Linux server.

• When the Roundcube interface is accessed via http://webmail.example.com, the following error is shown in the menu Settings > Filters:

  Unable to connect to Sieve server

• One of the following errors can be found in the file /var/log/plesk-roundcube/errors:

  ERROR: Connection refused (111)

  OR

  ERROR: Not currently connected (1)

  OR

  PHP Error: Unable to connect to managesieve on localhost:4190

• Courier-IMAP is shown as the IMAP/POP3 server in Tools & Settings > Mail Server Settings.

Cause

To create filters in Roundcube, the IMAP server should support Sieve.

Courier-IMAP does not support Sieve.

Resolution

Install Dovecot as IMAP/POP3 server using the instructions from the article How to install / uninstall Plesk components?

Note: Dovecot configuration used by Plesk is compatible with Courier and acts as a drop-in replacement.