Is there a way to be notified when mailboxes reach full status so that I can be proactive in either increasing the limit or notifying the mailbox holder before they are unable to receive e-mails?
The solution here is to have a clear understanding of the different quotas available within Exchange and especially an understanding of the client experience as each of the relevant quotas is exceeded.
There are three quotas that can be set on any given mailbox: Issue Warning, Prohibit Send and Prohibit Send and Receive. I'll give a description of each quota and how it works.
The Issue Warning quota will send an e-mail to any mailbox that exceeds the quota set for that mailbox. For example, if my Sengupta, David mailbox is set with an Issue Warning quota of 100 MB, I will receive a warning message such as the following once my mailbox exceeds 100 MB in size:
Your mailbox has exceeded one or more size limits set by your administrator.
Your mailbox size is 107079 KB.
Mailbox size limits:
You will receive a warning when your mailbox reaches 102400 KB.You may not be able to send or receive new mail until you reduce your mailbox size. To make more space available, delete any items that you are no longer using or move them to your personal folder file (.pst). Items in all of your mailbox folders including the Deleted Items and Sent Items folders count against your size limit. You must empty the Deleted Items folder after deleting items or the space will not be freed. See client Help for more information.
This warning message is not sent instantly, however it is sent during the IS maintenance window that has been configured for the store containing my mailbox...typically every night.
The Prohibit Send quota acts similarly, however it will block a user from being able to send messages until he cleans up his mailbox.
The Prohibit Send and Receive quota does just what it says: it prohibits a user from both sending and receiving messages once the quota is exceeded. Anyone attempting to e-mail this user will get an nondeliverable message stating that the user's mailbox is full.
Based on what you've written, my hunch is that you only have a Prohibit Send quota set on your mailboxes, so users aren't getting a warning before their Prohibit Send quota kicks in.
Here is what I recommend you do: Set up an Issue Warning quota for users that is at about 90% of the Prohibit Send quota. This should give users sufficient warning prior to being locked down from being able to send mail. I'd also recommend you set a Prohibit Send and Receive quota that's set to something like 200% of your Prohibit Send quota. This typically provides users with sufficient quota room to be able to reduce the size of their mailbox from the Prohibit Send quota without bouncing mail because their mailbox is full (which I find unprofessional).
You might wonder why you should set the Prohibit Send and Receive quota at all. Simply put, if you don't use this quota on mailboxes in your environment, you're leaving yourself wide open to certain kinds of mail loops or denial of service attacks in which these mailboxes could expand rapidly to fill the drive that they are stored on. For example, if you don't have this limit set on your mailbox and if I set up a mail storm utility to send you a 1 MB attachment every 10 seconds, your mailbox would grow at approx. 8.4 GB/day until the server's disk filled up and your Information Store service shuts down.