Overloading Microsoft Outlook mailbox capacity

If you have a 50Gig mailbox, you also have a 50Gig Online archive. Learn to archive so you do not reach the limit. If you reach the limit on both, then go get a Exchange Online (Plan 2) stand alone license, and upgrade your account to that exg license


Capacity alerts
Exchange Online provides three kinds of notifications when a user's mailbox is nearing, or at, capacity:

Warning: The user receives an email warning that the mailbox is approaching the maximum size limit. This warning is intended to encourage users to delete unwanted mail.

Prohibit Send: The user receives a prohibit-send notification email when the mailbox size limit is reached. The user can't send new messages until enough email is deleted to bring the mailbox below the size limit.

Prohibit Send/Receive: Exchange Online rejects any incoming mail when the mailbox size limit is reached, and sends a non-delivery report (NDR) to the sender. The sender has the option to try resending the mail later. To receive messages again, the user must delete email until the mailbox is below the size limit.

https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#capacity-alerts