If enabled, users automatically receive a Quota Warning Letter once every 7 days when their mailbox utilizes 90% or more of its total storage capacity.
The default Quota Warning Letter is:
Subject: Your Mailbox Is Almost Full
Your mailbox is almost full.
Please delete some messages and empty your Trash folder to make more space. If you allow your mailbox to become completely full, you will not be able to receive new messages.
Please check the appropriate Tutorial Guide on how to change the letter.