In Alentus ExchangeMail, is a notification sent out when an email client account approaches the disk space quota for the domain?

Yes.
You will receive a notification according to what is set in "Exchange Storage"

You can change this setting when you login to the Exchange Control Panel (http://panel.alentus-exchange.com/)
It is located under the 'Accounts' Tab.
Choose the "Select Operation" dropdown for the desired account.
Click on "Change Storage"

You should then see the following:
Exchange Storage
for user@example.com

Current Mailbox Size: #MB
Current Warning Limit: #MB
Current Storage Quota: #MB
Change Storage Quota to (MB):
Set Warning Limit:
*If you exceed your storage quota you will not be able to send or receive any messages

Change what you need and click 'Update'

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