We're in the process of upgrading Dovecot from version 2.1.17 to 2.2.27 (package Debian Stretch). We have a shared NFS storage in use by multiple servers without Dovecot director. I've tested by connecting to IMAP on the new server to my own mailbox. This isn't a problem. But when the server with the Dovecot version 2.1.17 LDA drops a new message in my mailbox it thinks the index file is corrupted, and the message bounces to it's sender. So there must be a index compatibility issue between both versions. This is still no problem if I upgrade the entire platform in a maintenance window at once. But what I want is a rollback possibility. To which minimal Dovecot version should I upgrade the old platform to create this rollback possibility.

I was looking for a table of dovecot index compatibility between the different versions (forward and backward), but couldn't find it. Is there any? Please advice.

Best regards, Frido