I have two systems. Up to date installation of UCS mail server with dovecot 2.2.13 and server courier imap which has all the user emails.
I tried this in my homelab environment (but not univention) and worked great. However I cannot make it work in univention. I get error:
sudo doveadm -Dv -o mail_fsync=never -o imapc_host=oldserver.com -o imapc_user=user.name -o imapc_password=password -o imapc_features=rfc822.size -o imapc_list_prefix=INBOX sync imapc:
I even tried not to use INBOX namespace.
But I get error
dsync(root): Panic: file quota-maildir.c: line 827: unreached
I tried investigating but could not find any help, the only thread on dovecot mailing list was left without answer. Last resort to get help from dovecot IRC channel but folks pointed to old version. (2.2.13 was released in 2014, 2.2.27 was released 7 months ago)
dsync migration would have been much easier and transparent, users would be able to keep using emails and once sync is done, just switch to new account. However it is not working and I would need to go and use perl migration script it seems.
I tried to disable quota plugin on dovecot, then imap sync started, but stalled. And client mails stopped working with dovecot. So no actual migration happened.