MapiExceptionMessagePerFolderCountQuotaExceeded

The other day during a mailbox migration of an on-prem Exchange 2013 mailbox to Exchange Online, I came across the following error:

Error: QuotaExceededException/MapiExceptionMessagePerFolderCountQuotaExceeded: Error: Cannot save changes made to an item to store. --> MapiExceptionMessagePerFolderCountQuotaExceeded: Unable to save changes.

The message was due to a folder in the user’s mailbox which had almost 1,500,000 items in it. The problem is that Exchange Online enforces a maximum number of 1M messages per mailbox folder limit (across all Exchange Online plans). New messages can’t be delivered or saved in a folder when this limit is reached.

There is also a warning for number of messages per mailbox folder. Once a folder in Exchange Online reaches 900,000 items, Exchange Online sends a warning message to the mailbox owner. When this quota is reached, warning messages are sent once a day.

To check which folder(s) in a user’s mailbox is causing this, you can use the Get-MailboxFolderStatistics cmdlet:

Get-MailboxFolderStatistics "user" | % {If ($_.ItemsInFolder -ge 1000000) {Write-Host "Folder with $("{0:N0}" -f $($_.ItemsInFolder)) items: '$($_.FolderPath)'" -ForegroundColor Red}}

It is also crucial to check other limits before starting to sync mailboxes, like mailbox size limit, folder limit, and so on.

Hope it helps!