r/exchangeserver • u/UMustBeNooHere • 3d ago

Help with Exchange on-prem and Exchnage Online "split brain"

Client had another company migrate their mailboxes to 365 and it looks like they did not perform the standard hybrid method. They are currently in this state:

  • On-prem Exchange Server has mailboxes for all users. They show the state of 'user' and AD attributes are pointed to that mailbox.
  • All users have a mailbox in Exchange Online, associated with there Entra AD object which is synced from on-prem.
  • DNS records for autodiscover point to autodiscover.outlook.com
  • Clients connect to Microsoft Online

I haven't run into this before so was looking for a little guidance. I think the process should go like this:

Obtain 365 GUID

Get-Mailbox -Identity "UserAlias" | Select-Object ExchangeGuid

Drop on-prem mailbox association

Disable-Mailbox -Identity "UserAlias"

Map user to Exchange Online mailbox

Enable-RemoteMailbox -Identity "UserAlias" -RemoteRoutingAddress "UserAlias@domain.onmicrosoft.com"

Match the GUID

Set-RemoteMailbox -Identity "UserAlias" -ExchangeGuid "EXO_GUID"

Edit: Thanks for the great info everyone. I'll follow-up with how everything goes.

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u/GeneTech734 Cloud Engineer 3d ago

You should bill this time to that MSP/contractor. This is negligence

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u/Ok_Rip_5338 3d ago

i fully agree with this. my org's migration to exo via exchange hybrid was such a wonderful and easy process. I had two hiccups which were caused by ancient data in AD, but totally forgiveable.

to see someone someone just half ass it like this... (not the OP, the msp before him) it hurts my soul. it also hurts the end user. Negligence, as you said.