Install dovecot-sieve by entering the following commands in the terminal:
sudo apt update sudo apt install dovecot-sieve
Description:
secure POP3/IMAP server - Sieve filters support
Dovecot is a mail server whose major goals are security and extreme reliability. It tries very hard to handle all error conditions and verify that all data is valid, making it nearly impossible to crash. It supports mbox/Maildir and its own dbox/mdbox formats, and should also be pretty fast, extensible, and portable. . This package provides Sieve filters support for Dovecot.
Homepage: http://dovecot.org/
Version: 1:2.2.33.2-1ubuntu4
Section: mail