I’m aware that this thread is rather old, but this is great advise @esackbauer and much appreciated. It is not very well documented elsewhere and this would be good to pickup in the official documentation.
I have successfully tested this on a few migrations currently from external systems towards mailcow (the goal of course). Provided that the external system also uses postfix (which is quite common) it is a good idea to add there also transport maps for each individual user so that as soon as that user is migrated, their email address transports to the mailcow server. Hence we can do a transition without haste easily and one-by-one (adding transport map entries for each user just migrated to the old system and deleting them on the mailcow level). This also ensures “internal” mail between migrated and non-migrated users continues to flow perfectly without nobody noticing they are on different servers. That is for mail of course and not calendar or any other sharing. So always a good idea to speed up migrations where that is needed.
Just wanted to add this here and mayeb this can be a good base to a more complete migration doc.