From kaspersky.co.uk
Outdated and completely unsupported versions of Exchange Servers pose an undeniable danger to corporate infrastructure and to mail flow. However, many administrators still believe in the proverb “if it ain’t broke — don’t fix it”, and prefer not to update Exchange unless absolutely necessary. And this appears to be why Microsoft decided to develop its transport-based enforcement System for Exchange Online.
The main purpose of this system is to notify administrators that they’re working with outdated and possibly unsafe software, and that, if they don’t subsequently update in a timely fashion, mail delivery from vulnerable servers will be gradually throttled and eventually blocked. It’s hoped that this system will serve as a convincing reason for administrators to finally upgrade or update Exchange Servers.