Exchange 2010 – Office 365 Migration: Handling Shared, Meeting Rooms, and DL

DISCLAIMER: While the below "should" work...I'd be cautious about undertaking it as your "backup" solution, as that's not what it is designed for.

Because you haven't setup O365 at all, in theory you could do the following, assuming you plan on going to O365 within 60 days (I believe that's the limit):

• Setup O365 for a cutover migration (you need to really understand what a cutover migration is, how it works, etc. before proceeding) but don't actually DO the cutover. Just the staging part. You'll setup the TXT record for the domain, and then setup the Exchange migration sync part.
• Start the sync and just let it sync each day for each mailbox
• DON'T, repeat DON'T finalize the cutover migration in the portal until you are fully ready to cutover...as once you do syncing will stop.

Note: this only handles mailboxes, not public folders or anything like that.

Basically what you'll be doing is a cutover migration but you'll be taking your time doing it. If all mailboxes sync within a few days, then at that point if your Exchange server on AWS fails, you could basically force the finalizing of the cutover quickly (MX records at this point, finish the cutover wizard, etc.) and retain all the email up to the last sync (happens every 24 hours or manually).

That's about the best I can think of.

I've the same problem in my domain. Someone manually create the o365 mailbox for users who already have an on-premise mailbox

I've found this way to fix it:

• Export office 365 mailbox in PST
• Remove office 365 user license (this will remove his cloud mailbox)
• Remove office 365 user from office 365 AD:
  • Remove-MsolUser -UserPrincipalName youruser@youroffice365domain.com -Force
  • Remove-MsolUser -UserPrincipalName youruser@youroffice365domain.com -RemoveFromRecycleBin -Force
• DirSync (recreate user in office 365 AD)
• Reassign the office 365 license for the user
• Migrate user to office 365
• Restore PST

I think is more simple and straightforward. You can also re-migrate your mailbox on-prem (offboarding) if you need it.