The integration of the Cyrus IMAP and POP3 server has been discontinued with UCS 4.3.
That means that while Debians Cyrus packages can still be installed, they are not longer integrated into the identity management system: changes to the user database will not trigger creation, modification or deletion of Cyrus mailboxes and Postfix is not configured anymore to work in conjunction with Cyrus.
Users of Cyrus on UCS are encouraged to migrate to the Dovecot IMAP and POP3 server. The integration of Dovecot has reached functional equality and will be the only one supported from UCS 4.3 on. A migration guide from Cyrus 2.4 to Dovecot can be found here: https://help.univention.com/t/6679
As part of the cleanup following the removal of Cyrus from UCS, the
mailquota policy has been removed. The Dovecot integration requires the quota information be set directly on the user object (see UCS manual).
An upgrade from UCS 4.2 to UCS 4.3 will be prevented as long as the package
univention-mail-cyrus is installed. A UCR variable can be set prior to starting the upgrade, to circumvent this check and upgrade to UCS 4.3 without removing the Cyrus integration:
ucr set update43/ignore_cyrus_check=yes
Be advised that the use of Cyrus under UCS 4.3 is not supported by Univention. Such a system may not be able to store received email, even if Cyrus is still running, as the legacy Cyrus integration will not create mailboxes for new users and Postfix may not be configured to deliver emails to Cyrus anymore. Of course no data will be lost, as existing mailboxes will not be touched.