Following up on my own post - partial potential solution, and additional questions.

So, it *looks* like using something like "auth_username_format = %Ln" on the old server dovecot config will fix the problem I outlined below. (I still need to try it, and since it's a production server, I need to be careful when/how I do that.)

But this doesn't solve a similar issue I can certainly see happening.
Lets say, on the new server and old server, I'm actually syncing different accounts.
Say on the old server it's joes
But on the new server, it's sallys
(Joe decided they'd rather be called Sally.)

So, I'm trying to sync the joes mailbox on the old system with the sallys mailbox on the new one.

Is there a way that handles that?
And if so, it would also allow me to solve this with the problem outlined in the prior post by supplying the source and destination mailboxes individually / separately.

Thanks again!
-Greg

I'm, trying to sync/backup mailboxes from a current dovecot instance to a new server. But the old server has real system accounts. The new server has virtual users. And the account/user-name is different. Example: Joe Sixpack is joes (on the old system) but is joes@sixpack.org (on the new system.) I'm using something like this to attempt the sync doveadm  -o mail_fsync=never backup -R -u joes@sixpack.org imapc: This works from the new system, as it recognizes the user with a full @domain.com username - but the old system doesn't know who joes@sixpack.org is - and just wants joes. I don't see a way to specify the remote account username independently from the local one. What's the best way to handle this? TIA -Greg