Depending on your storage quota settings for the mailbox one of 3 following things can happen:

Issue Warning: set size limit
Prohibit Send: set size limit
Prohibit Send and Receive: set size limit

Check your storage quota settings for the target mailbox and see what the settings are.

Also make sure that Quota Message Service for Exchange is installed and setup.
Here is a good article on how to do it:
http://www.msexchange.org/articles/customizing-mailbox-limit-warnings-quota-message-service.html

Also user would get a notification in OWA that the mailbox is over limit and unable to send.

If the email was sent from outlook, the email would bounce back with notification that they need to contact system administrator or free up space.

Also any emails which been sent to them would bounce back with a note that user ran out of space in mailbox if exchange was set to prohibit both send and receive.

Bit hard to believe that user would not notice.

Hi LondonCitizen,

To be more clear on my question, if the prohibit send limit is exceeded, and the user received but did not read the system email stating that, and then sent an email via the OWA interface sometime later, they would receive another system notification? This is important because the exchange server configuration is being blamed for a lapse in communication between my company and a customer.

The user has not received any additional notifications upon trying to send emails, only the original one that the mailbox was over the limit.

This user receives alot of email traffic and was distracted by a personal issue. I really need to know if she should have received a warning for every send attempt?

Thanks so much for your input!

Thanks so much.

Please see the picture how notification would be displayed in OWA.
Even with the notification user can still click send button and send mail which will fail delivery and would generate bounce back notification.

Please see:
http://kb.mit.edu/confluence/display/ist/What+happens+when+a+mailbox+goes+over+quota

This is great information, many thanks! We are stuck using Exchange 2003 and the 2003 OWA interface does not have the mail box display as shown in the screen shots however. I guess this can be additional leverage for pomoting an upgrade. Also, in Exchange 2003 notifications do not become part of the normal email log and can not be searched for using the search tool in System Manager, sort of strange. My server is not sending failure messages per each sending attempt after the first "email sending prohibited" message is sent out. Any ideas why? Again, thinks very much!

Hello, I will have a look, strange behaviour might be default for exchange 2003, I'm with exchange 2007 at the moment, still remember the times with exchange 2003 tho.

I will get back to you with more info.
Cheers.