Outlook accounts can have up to 100GB of storage per mailbox, after which you may be unable to send and receive email.

"Quota Exceeded, your account is above quota limits"

If your mailbox is nearing its maximum capacity, we will begin sending you emails and in-app messages telling you your storage is almost full. If it does exceed capacity, you won't be able to send or receive any messages. Also, people who send you email will receive an error message that tells you your mailbox is full.