Hello,
I have upgraded our cyrus IMAP infrastructure last year from version 2.3.16 to version 3.4.x. During the process I also replaced our previous servers that were on RHEL 6 by new servers on RHEL 8.
We have a MURDER setup an we had altnamespace set to on before the upgrade. This parameter had been set to on since our first Cyrus IMAP installation more than two decades ago because most of our mail clients use thunderbird. We also have unixherarchysep set to off.
I started by tranfering our mailboxes from our initial backends to new backends on RHEL8 and Cyrus IMAP 2.4.22, then once all our frontends had been upgraded to Cyrus IMAP 2.4.22, I transfered all of our mailboxes again to some other backends on RHEL 8 and Cyrus IMAP 3.4.5. Then I upgraded all our frontends and previous backends to v3.4.5.
This procedure had been tested in lab before the upgrade of our production servers. I decided to go for Cyrus IMAP 3.4.x instead of version 3.0.7 that is packaged by RedHat because of issues. I did also package version 3.0.18 but I did not resolve my issues.
I did not want to upgrade to version 3.6.x or 3.8.x because the organization of the mailboxes directories by UUID would complicate our mailboxes maintenance tasks ; mostly mailboxes restorations from backups.
Since our upgrade I also saw, like you did, that some of our user's mailboxes had a new INBOX sub folder. I did not found the reason, but I wouldn't say that it is related to Thunderbird has most of our users use this MUA and that this folder has been created only for a few users ; mostly those that may choose an alternative MUA.
Regards,
Stéphane
