Here is what you need to do. say you have 2 servers and both IMAIL and Merak support IMAP. setup Merak mail server but do not make it the primary MX record.
Then enable the server and create the propert usernames and passwords.
then say for example your domain name is mylinuxserver.com
setup a hostname for this mail server such as
then contact your users and give them the username and passwords for their account and have them configure the account on outlook , thunderbird or whatever email they use.
Ask them to configure their new accounts with IMAP and to login to the new server.
Once they have configured this accounts on their email programs they will have two accounts configured on their email clients.
They will have the IMAL account and the Merak server
Write instructions on how to drag email between the two servers and they will be able to move their email themselves. it is just a matter of dragging emails from one server into the other.
They can actually create folders inside the Merak server so they can manage their mail properly. Tell them that they have 30 days to start moving their email to the new server. Then after 30 days change your MX record and point all new email to the MERAK server and then pull the plug on the old server after 10 days.
This will give them enough time to migrate their old boxes , if they are lazy then that is their fault you gave them enough time to do it.
This is how we at my old ISP moved 20,000 mail boxes and everyone was happy with the new server and were able to keep their old mail.