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Our company's email is hosted Exchange 2016 (hosted by AppRiver). It's a simple setup: only ~100 mailboxes, single domain, no linkage between Exchange and our corporate file/print/login AD, no public folders, etc.

Our company was just acquired by another company that uses Office 365 (managed by a reseller). The acquirer, although larger than us, is also pretty small: under 200 users.

Now we need to figure out the best path to get our email/calendar/contacts integrated with the acquirer. The desired end-state:

  • all mailboxes end up on the acquirer's O365
  • emails sent to our "old" email addresses end up in the right user's mailbox
  • all emails sent from our company use the acquirer's email addresses, even if they were sent to our old email addresses.
  • calendars, mail, and contacts and mail continue to work as before for end-users with relatively little user training or disruption. For example, requiring users to delete and re-add their email accounts in outlook and phones is OK. But users having to re-create all calendar invites and contacts would not be OK. Email being unavailable for a weekend is OK, but not OK if it's a full week.
  • we can stop paying AppRiver within a few weeks after migration is complete.
  • we don't need to do a phased migration-- it's OK to migrate all mailboxes at once.

The options I've heard so far are:

  1. Create new mailboxes in the acquirer's O365 account, and then migrate mail and calendar info from one mailbox to the other.
  2. Migrate mailboxes directly from hosted exchange at AppRiver to O365
  3. Migrate from AppRiver's hosted exchange product to O365 resold by AppRiver. Then migrate from AppRiver O365 to the acquirer's O365.

What are the pros/cons of each of these options? Will one of them be much easier/better than the others? Is there another option that's better than all of above?

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I'd be curious to read about the reasons behind some of the suggestions that you have heard. Option 2 is the most straight forward. Option 1 and 3 are both completely un-necessary.

This process is quite simple actually.

  1. In the new organization's Office 365 account add/verify the domain of the old organization.
  2. In the new organization's Office 365 account start an exchange cutover migration and migrate the mailboxes from the old organization. (You would want to make sure there were no conflicting usernames before doing this - even if the actual e-mail address is different. The part in front of the @domain.com should not conflict.) Leave the migration task active - it will periodically resync new messages.
  3. Assign appropriate licenses to the newly created users
  4. Use powershell to find and configure the new mailboxes so that the new mailboxes have a primary SMTP address and login of the new domain while keeping the old domain/email address as an alias.
  5. Update the old domain's DNS records to the suggested Office 365 settings so that mail flows in to the new domain.
  6. Remove e-mail profiles from user computers and devices and re-add them using the new e-mail/password. There are other ways around this, but it can be quite problematic.
  7. After you are sure the old DNS records TTL has expired, perform a final sync of the migration task, and close out the migration (stop it and delete it) to finalize the mailboxes. This will grab any remaining messages left in the old mailboxes.
  8. Add the new users to whatever distribution and security groups they need to be in for the new organization.
  9. Once you are sure your new mailboxes are working properly and contain all the data, you can cancel the old service.

If something goes wrong after step 2, you can easily use powershell to find and delete the newly created mailboxes/users with some carefully crafted commands. You can stop, delete and re-create the migration tasks too, with little impact. You can re-run the migration task on the same mailboxes - it doesn't duplicate the mail. So, you have quite a bit of flexibility.

Also, don't forget that you have fairly decent support from Microsoft when you have Office 365 business accounts. So, you could also discuss this with them first or during the process.

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