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
ERROR: Connection refused (111)
ERROR: Not currently connected (1)
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.
To create filters in Roundcube, the IMAP server should support Sieve.
Courier-IMAP does not support Sieve.
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.