Hitting the Million Messages Limit in an Aggregate Group Mailbox
A curious problem happened when a mailbox reported hitting a folder item limit (one million items). The mailbox was an aggregate group mailbox, a system mailbox used to make it easier to search Microsoft 365 Groups. Microsoft now uses a different method to search group mailboxes and will remove these arbitration mailboxes by the end of 2021. If you meet the problem, use a mail flow rule to stop messages being delivered to the mailbox.