Exchange 2010/2007 provides you the option to send the custom system messages. You can customize the message which users get when his mailbox quota is getting to close to the limits. Default warning message does give some information. If you wanted to add more data like steps to delete unwanted email, link to your internal email policy etc. These things can be done easily with new-systemmessage or set-systemmessage

Below is the example of the same. Remove-systemmessage removes the existing default warning message.

$K has all the data which you wanted to be in the warning message, it has link, message with different bold and colour. You can include any basic HTML stuff to format the data.

New-systemmessage helps us to assigning the HTML data into the system for warning mailbox

Remove-SystemMessage En\WarningMailbox$K=@"<BR><b> <font Size="3" color="RED" Face="Arial"> You will have problem sending email if you do not delete your unwated emil</b> </Font></BR> <br>Make sure that you delete your unwated and deleted items from your mailbox </br><br> <A name='Internalportal'>Company email policy</A></BR>"@New-SystemMessage -QuotaMessageType WarningMailbox -Language En -Text $K