A mailbox receives “The recipient’s mailbox is full and can’t accept messages now” or “The message store has reached its maximum size.”

Scenario: Customer emails a recipient and receives this error message:
The recipient’s mailbox is full and can’t accept messages now. Please try resending this message later, or contact the recipient directly.#554-5.2.2 mailbox full 554 5.2.2 
STOREDRV.Deliver.Exception:QuotaExceededException.MapiExceptionShutoffQuotaExceeded; Failed to process message due to a permanent exception with message Move/Copy messages failed. 16.55847:4000000 

The sender may also receive the following error message in Outlook: 
The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, permanently (Shift + Del) delete them. 

The sender may also receive the following error message in Outlook Web App: 
The action couldn’t be completed. An error occurred on the server.  

Cause: This occurs if the items in the Recoverable Items folder of the recipient have exceeded the default quota of 30 gigabytes (GB).  

Resolution: Administrator needs to increase recoverable items quota for the recipients by using this one liner: set-mailbox -recoverableitemsquota 50GB -UseDatabaseQuotaDefaults $false