We have the following situation: We are using Dovecot on several Mailservers administered via 

ISPConfig. The servers all use old versions of Debian and ISPConfig and my job - I joined the company as a Linux system administrator 6 months ago - is to consolidate these old servers into one new server I already set up. I did a trial run with one user from one company by way of duplicating her account on the new server, creating a new mail account in her outlook and changing the MX record from the old to the new server. This works. This was rather time-intensive and I still have to face the problem of what to do with the existing mail data once we shut down the old servers one-by-one. Is there a way to move or even better integrate the data from the old mailbox on the old server to the new mailbox on the new server without any user intervention? 

https://pypi.org/project/dovecot-archive/ looks promising, but it assumes a situation we don't have: In our case the original user and the user in whose mailbox the data should be stored have the same email address. I am very open to suggestions on how to do this the smart way. Even manually entering the users' account data by hand on the new server is very time consuming. At the same time this is a much needed opportunity to weed out old mailboxes and forwardings which are not used any more. Even complete domains on the old servers have proven to be not in use anymore - the client company switched to Exchange online according to the MX record and failed to inform us. Anyway, this is a huge load of work and I'd rather spend more time preparing correctly and efficiently repeating simple tasks again and again. I am grateful for any advice. After all, this situation is certainly not unique.

Yours faithfully

Stefan Malte Schumacher