I currently have a Debian server setup with Exim4 and Dovecot which handles mail for 7 domains. Currently mail is pumped into the local mailboxes, so if a user "a" exists then all mail to a@domain1.com a@domain2.com...etc get delivered into his mailbox, also I have multiple aliases setup. I also want to reduce the user count for specific domains, so user "a" can get mail to a@domain1 and a@domain2 but not a@domain3...etc

I looked at the online demo and found that Zimbra is an amazing product and want to install it on a new server I am setting up. I will be using Zimbra 5.0GA on Ubuntu 6.06 LTS

I have the following questions/worries regarding the migration:
1/ I need to migrate ALL mailboxes and folders to the new Zimbra server along with the passwords
2/ Need to accept and send mail for ALL of the previous domains
3/ Need to do this with minimal downtime of mail services

If I modify the MX records of the domains while Zimbra is not setup correctly incoming mail will be lost (since they will be pointing to a new IP). How can I overcome this issue and join the ever growing satisfied user base of Zimbra?

I am setting it up locally before it is moved to the sever room, so no real IP will be present only once it makes it into production.

For the migration I read about imapsync, however I think using the brute force approach of downloading to Thunderbird clients and copying back to the server the most reliable, unless I am mistaken.

Please help me on this topic,