We just detected, that under some circumstances Courier-IMAP will generate a new UIDVALIDITY for every access to an IMAP-folder, if the folder had been created by a simple "mkdir" and not with maildirmake. Courier doesn't save his uidvalidity-file in this case.

Courier DOES generate a sticky UIDVALIDITY after the first "real" access by a User through the IMAP-SELECT-command. That's why this problem isn't a "real" problem in the world. Courier does NOT create this IMAP-UIDVALIDITY if the user just uses POP3-access, because there the UIDVALIDITY isn't used.

We have a migration scenario where we have many users with POP3-only access (so that they DO have many mails in der INBOX) and an incremental migration trough doveadm/dsync/imapc fails (caused by consequently changing UIDVALIDITYs).

doveadm just uses the IMAP EXAMINE command to open the source IMAP-folder on the Courier's side readonly. Courier DOES handle this access "readonly" and doesn't consequently doesn't generate a UIDVALIDITY-file. Instead Courier uses random UIDVALIDITYS by every access and doveadm always has to copy all mails (and duplicates them if "-1 -R" is used!).

To fix this problem and to make a migration and/or doveadm backup/sync much more reliable Dovecot should open the source folder with a SELECT command instead.

(Maybe one SELECT and then change to EXAMINE, if you think EXAMINE would be more safe.)

Maybe this is also relevant for a real permanent imapc-situation, where Dovecot/imapc is used to fix broken IMAP-implementations from an older backend.

Peer

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