On Aug 14, 2019, at 1:26 PM, Timo Sirainen via dovecot <dovecot@dovecot.org> wrote:

It probably has something to do with using mbox format. Are the IMAP UIDs changing unexpectedly? Errors/warnings logged related to it? Unfortunately it's a rather troublesome mailbox format. There are likely some bugs in Dovecot mbox code, but it's difficult and time consuming to try to reproduce any of the bugs so I've mostly given up trying.

I'm not getting any errors or warnings as far as I can tell, and I don't think the UIDs are changing unexpectedly -- messages are not getting re-downloaded randomly. That is, everything SEEMS to be working fine, as far as I can tell.

So many people still use mbox that I hope we can fix this issue.

I'm happy to help test or provide further debug output... this problem is certainly reproducible here, and it seems like lbutlr has a similar problem, so hopefully we can address at least this one...

(I'm also happy to give you the Junk mailbox and index files... there's nothing sensitive in my spam!)

Cheers.

--- Amir