Cyrus Imap Sysadmins Beware, Debian Bookworm Might Hose Your Mail
A warning for all sysadmin using Cyrus imap on Debian Bullseye, don't switch to Debian Bookworm just yet. Your users might lose access to their email. (Updated on 2023-07-19)
You will want to carefully investigate Debian Bug 1037346.
Update - the bugreport now includes a remark from the packagers:
the patch needed to update to 3.6 is included in Debian version
3.2.6-2+deb11u2.For those that have already switched: read (and then do) this.
You also want to read the Cyrus manual on the recommended way to upgrade, here.
AFAICT, this shows that you can not use Debian bullseye-backports either.
You might be able to avoid problems by selecting the right version from Debian Snapshot.