Lately we’ve done a few migrations for clients who have their email currently hosted by a Microsoft Exchange hosting provider and want to move to Microsoft Exchange Online. These migrations have presented a few unique challenges, so we thought we’d share our experiences.

How to gain access to the mailboxes

One of the biggest challenges you face right from the start is determing how to gain access to the mailboxes. Ideally you’d have an account which has access to all your organization’s mailboxes on the source server. Some hosting providers can set that up, others can’t or won’t.

If you are unable to get an account which has access to all the mailboxes you need to migrate, you will need to know all the individual user’s passwords to gain access to their mailbox. This can typically be accomplished in one of two ways. You can either ask the users for their password or you can reset the user’s passwords, so you know what the password is set to.

If you have a very small number of users, you might be able to round up all the passwords, but in most cases, you’ll probably need to develop a migration plan which resets the users when they are migrated. In some cases, this could have the added benefit of denying the user access to the source mailbox after they have been migrated.


The next challenge you face is coexistence. Again, a lot of things depend on what is possible with your current hoster, in this case it’s whether or not they’ll let you setup forwarders on the mailboxes. If they do let you setup forwarders, you’ll typically need to create contact objects which have a target address of [email protected]

Right before you get ready to migrate a user, you will go in and manually set the forwarder on their account and then begin the migration. This ensures all new mail sent to their mailbox will be delivered to their Microsoft Online Mailbox. If they don’t let you setup forwarders, it gets a little trickier. Basically that all but forces you to do a one time cutover. If you don’t do a one time cutover, mail sent from an unmigrated user to a migrated user will be delivered to the migrated user’s mailbox on the source server since forwarding is not in place.

Data Migration

If you are going to do a one time cutover it’s very important that you test the throughput of the migration tools prior to the migration. You might discover that’s it’s simply not possible to migrate all data over the course of a weekend and you instead need to migrate a subset of data, such as calendar and contacts. Once those items are complete you can begin moving the mail data in stages.

Maybe the first run would be all mail newer than 7 days. The second run new mail newer than 30 days. The final run everything old than 30 days. Other options include having users archive more of their mailbox to PST. If your bandwidth seems to be the bottleneck(mail has to be copied from the hosting provider to the migration workstation and then to Microsoft Exchange Online) contact MessageOps about using our datacenter access to host your migration workstations so the traffic goes from the current hosting provider to the datacenter and then to Microsoft Online… the traffic doesn’t have to go over your local network.

Another consideration if doing a one time cutover is the OST rebuild. If you migrate all the users over a weekend and they are in cached mode, on Monday morning they are all going to have to download all their mail to rebuild their Outlook OST file. This can often times bring an organization’s internet connection to a halt.

These are just a few things to consider when moving from your current hosted Exchange environment to Microsoft Exchange Online. If you have any questions or would like free assistance with your migration, please contact us.

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