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Mailbox present in O365 but missing on-prem
13-Nov-23

When trying to set a retention policy on a mailbox I came across an issue where the O365 mailbox could no longer be viewed in ECP on-prem as a remote mailbox. I'm not sure how this occurred but thankfully it can be recovered.

Exchange 2016
Enable-RemoteMailbox 'SoapJ' -RemoteRoutingAddress 'SoapJ@contoso.mail.onmicrosoft.com'

EXO
Get-Mailbox "Joe.Soap@contoso.com" | fl *ExchangeGUID*

Exchange 2016
Set-RemoteMailbox 'SoapJ' -ExchangeGuid 'Exchange guid value of Exchange Online mailbox'


Reference: https://learn.microsoft.com/en-us/exchange/troubleshoot/user-and-shared-mailboxes/mailbox-exists-exo-onpremises