Cannot Send Mail - Your mailbox is full

Cannot Send Mail - Your mailbox is full This is the message several people are getting that I work with. We have check their email quota and it so OK. The attachment is one picture or a 2 page PDF file that are within the 25mb size limit. A simple signature is with this attachment. They are on the most current iOS 15.1 We have cleared their Safari cache, unloaded old apps, restarted the ipad and we are still getting the error. Most of the iPads are iPad Pros as well. We have removed the exchange account and added back and still the iPad mail app will not send email. Continue getting this error.

With Exchange accounts and from both cellular data & WIFI