Exchange System Mailboxes after migration to Exchange 365

Yes, it is possible to recreate the DiscoverySearchMailbox if necessary. The DiscoverySearchMailbox is a system mailbox in on-premises Exchange used for In-Place eDiscovery. If you remove it and later find the need to perform eDiscovery searches or if it's required for compliance reasons, you can recreate the DiscoverySearchMailbox.

To recreate the DiscoverySearchMailbox, you can use the New-MailboxSearch cmdlet in the Exchange Management Shell. Here's an example of the command:

New-MailboxSearch -Name "Discovery Search Mailbox" -InPlaceHoldEnabled $true -SourceMailboxes "email address removed for privacy reasons", "email address removed for privacy reasons" -StartDate "MM/DD/YYYY" -EndDate "MM/DD/YYYY"

Replace "email address removed for privacy reasons", "email address removed for privacy reasons" with the mailboxes you want to include in the search. Set the appropriate StartDate and EndDate for your search criteria.

Keep in mind that removing the DiscoverySearchMailbox means you'll lose any existing In-Place eDiscovery searches associated with it. Recreating it will start with a new mailbox and new searches.

Always ensure that you understand your organization's compliance and eDiscovery requirements before making such changes. If in doubt, consult with your IT or compliance team.