If mail says user has reached their inbox/outbox limit

Members click your mail tab than click either view message or compose

On the click side of your screen this will appear
* View Messages
* Compose

Folders

* Inbox (0)
* Sent Messages (1126)
* Trash (30)

you need to empty your folders

Go to drop bar on left and click all another box will appear click delete than ok.

Than go into your trash and delete all.

You will only be able to save 300 in your box.