“Remote Server returned '554 4.3.2 mailbox busy ...” error when you send mail to a mailbox in Microsoft Exchange Online

Applies to: Exchange Online


When you send email messages to a mailbox in Microsoft Exchange Online, the messages aren't delivered, and you receive the following error message:

Remote Server returned '554 4.3.2 mailbox busy; STOREDRV.Deliver.Exception:StoragePermanentException.MapiExceptionMaxObjsExceeded; Failed to process message due to a permanent exception with message Cannot complete delivery-time processing.



This problem may occur if the mailbox has reached the limit of items in a folder. In Exchange Online, the limit is one million.


To resolve this problem, move emails from the folder that contains one million items to another folder or to an archive.

More Information

To find the number of items within each folder in a mailbox, run the following PowerShell command:

Get-MailboxFolderStatistics -Identity <UPN> | FT Name, FolderPath, ItemsInFolder -AutoSize

For more information about the limits, see Exchange Online Limits.

Still need help? Go to Microsoft Community.