Sorry if the questiona has alredy been answered, but I couldn't find it and I'll be gratefull also for a big RTFM! if you tell me what is manual.

I have a mailserver (1 cpu 1,2 Ghz, 512 MB RAM and 40 GB disk) on Ubuntu 6.06 LTS with postfix 2.2.10 and dovecot 1.0 beta 3 and about 150 users. They access their maildir on the server with IMAP and clients like Outlook and Thunderbird.
Now I have a new server (a CentOS virtual machine with 2 vcpu 2,8 Ghz, 4 GB RAM and more than 100 GB of disk) and I'd like to take all users and e-mail and bring them to a new ZCS 6 server.

The mailserver should be the same (postfix), but I don't know users credential, they are managed like system users, so their password are encrypted in /etc/shadow and their emails are in /home/user_account/Maildir.

How do you advice to do to bring credentials and e-mails from the old server to the new, possibly without disturbing the users (it is they go home friday evening and on monday morning can use the e-mail without realizing that something changed?)

Thanks to replying people and best regards to everyone,