We come across this problem from time to time:-

Customer wrote: 
I never understand this as you set it to auto delete. 
Customer
--- Sent via BlackBerry
 
-----Original Message----- 
From: Quota Manager <Administrator@blackberry.com>
 Date: Tue,  x Dec 200x xx:xx:xx  To:customer 
Subject: Mailbox full 
One or more messages could not be delivered 
to your mailbox because they would have  put 
your mailbox disk usage over its quota. 
The system will keep trying to deliver  these messages.
To receive them, you must  delete some old messages
from your mailbox.  

---

Blackberry give you a 10Mb quota in the UK for your email, and unfortunately, even with "Auto-aging" set to 1 day, it seems that occasionally someone will send you a 9Mb attachment. Unfortunately the Blackberry web interface that Vodafone, O2 and T-Mobile use doesn't allow you to set the Auto-aging to 0 days. So you're kind of stuck in this situation.

This customer never seemed to get the grip of this, so I resorted to my ever favorite tactic, everyday analogy. Here it is.

You have a pint glass on the table.
You tell your butler to empty it once a day at 9am.
Every so often when a friend comes by they puts in a fluid ounce or two of nice wine from their vineyard.

As you can see, this system will work quite well most of the time. However, if one of your friends pops by and puts in almost a full pint in the glass, it is going to overflow before your butler comes along and empties it.

This is the situation. There are only two solutions.
i) Stop your friends from putting almost a full pint in the glass when they pass. (Ask them not to send large attachments)
ii) Get a bigger glass. (Ask your Blackberry provider for more storage)