Shared mailbox in Exchange Online is unexpectedly disconnected
In a hybrid environment, you migrate a regular mailbox from Microsoft Exchange Server to Exchange Online, and then convert the regular mailbox in Exchange Online to a shared mailbox. The shared mailbox in Exchange Online is unlicensed and doesn't have a hold applied. However, sometime after the conversion, the shared mailbox in Exchange Online becomes disconnected.
This issue occurs because the mailbox type in Exchange Online no longer matches the mailbox type of the on-premises remote mailbox. Because the direction of synchronization is from on-premises to Exchange Online, the mismatch in the mailbox type can cause Exchange Online to later revert the shared mailbox to a regular mailbox. Exchange Online might disconnect a regular mailbox if it has no license assigned and no hold applied.
Permanent loss of the mailbox contents can occur if these steps aren't completed within 30 days from the time that the mailbox was disconnected. If you assign a license after the 30-day period, the mailbox won't be reconnected, and a new mailbox will be provisioned instead.
To fix the issue, follow these steps:
Temporarily assign a license to reconnect the mailbox.
Update the on-premises remote mailbox to the
Sharedtype by running the following cmdlet in the on-premises Exchange Management Shell (EMS):
Set-RemoteMailbox -Identity <mailbox ID> -Type Shared
Convert the regular mailbox to a shared mailbox in Exchange Online by running the following command in Exchange Online PowerShell:
Set-Mailbox -Identity <mailbox ID> -Type Shared
Remove the temporary license.