Hi,

We currently have hosted Mail Service from Rediff for our 5000 users. The management wants to move on to On-premise Zimbra setup. All users are on a single domain and currently access mails through web-mail or POP account configured on MS Outlook. We do NOT have iMAP access to our mails.

For migration, the number of users are too large, and I do not want to mess-up things on Split-Domain with a hosted server where we have very limited access, and sice we do not have iMAP access to the server, we cannot use imapsync. Please advice if the following approach should be fine for migrating mailboxes (and aliases and distribution lists) ONLY (no calendars / contacts / tasks):

For Web-mail users,
1> Install Zimbra Desktop in each of the end user's computer and configure POP account for existing mailbox on Rediff. Users continue to use Zimbra Desktop for their Rediff mail account for some time
2> Setup Zimbra Server and create all 5000 user accounts on the server
3> Communicate a go-live date to all users, on which we'll migrate the MX pointer to our on-premise setup. End users will need to create a NEW ACCOUNT on the Zimbra Desktop to connect to ZCS (just create the account, mails will still be seen on their Rediff Account)
4> On a friday, change the MX and A pointers to the Zimbra setup, so that all mails now land our Zimbra server. Company web-mail will also open up the Zimbra web-mail page (since the A pointer is changed)
5> Ask users to start using the new Zimbra account on the Zimbra Desktop, instead of using the Rediff Account
6> Using Zimbra Desktop, copy mails from Rediff Account Folders to New Zimbra Account Folders using drag-and-drop
7> Delete Rediff Account from Zimbra Desktop

For Outlook Users
1> On the date of go-live, create an account on their Outlook for ZCS
2> Continue to use Outlook as a client for the new ZCS for the moment
3> Send an IT Support Person to each of the Outlook users one-by-one, install Zimbra Desktop on their computer and migrate PST to Zimbra Desktop
4> Henceforth, use Zimbra Desktop instead of MS Outlook

I would need expert advice on this approach of migration, since it would be a large instantaneous migration for web-mail users that contribute to 70% (rest 30% use Outlook).

Also, can we migrate multiple PSTs to Zimbra? And what if there were some mails existing on ZCS Server, and we dump a PST on that account, will it overwrite Zimbra Mailbox? Or preserve mails on ZCS and just add PST to it?

Thanks,
Sunny