I have a Debian Squeeze system running
I need you to migrate the 47 domains (there are 21 additional domains that are mine, that are just aliases; a few of the 47 domains are just aliases as well) with 612 email accounts to an up-to-date Debian (stable) system.
The source system uses mbox format and the new system will use maildir, so the files can't simply be rsynced, but need to be copied via imapsync or a similar tool.
It will probably be the best idea to move one domain at a time and evaluate whether the users are happy, etc. Once a domain or two is successful, the rest could go faster.
It will probably make sense to do an initial sync and make sure that it looks pretty good, and then I'll turn down the TTL on the DNS record, do a final sync, and switch the DNS record, and then verify everything looks good.
Users do not have direct access to change their passwords, etc. so no database modifications will happen behind your back, etc.
Most of my customers are on US/EDT timezone, so doing this change at night or weekends will be necessary. The domains that have customers not on east coast, are international, so there isn't a great time to do the migration, though weekends are better than weekdays.
The dovecot configuration will need to be updated. Postfix, sasl and postgresql are already up and running. I will copy the postgresql database over so you won't need to worry about that.