Please see Update on Transport Enforcement System in Exchange Online for an updated timeline.
2/20/2025: Please note that thottling/blocking has now been enabled for all versions of Exchange Server...
Updated Feb 20, 2025
Version 8.0
Blog Post
10 MIN READ
Throttling and Blocking Email from Persistently Vulnerable Exchange Servers to Exchange Online
davidvasta
Microsoft
MicrosoftWhile I don't think we have a definitive list published as to when EX2016/2019 will be throttled, I have been advising everyone I speak to have them removed from your environment before Oct 14th. All customers should be moving to Exchange SE as quickly as possible and removing all unsupported versions before the EOL date of Oct 14th. I would suspect that these will be blocked soon after that date and might be given the normal 90 days of grace but would not be surprised as they are not getting security updates and are considered "unpatched" and out of support.
Thanks for your answer. Of course I am aware and will do so "as soon as possible" but thats the thing:
I work at a managed service provider and we are currently upgrading all our directly, managed customers.
In the meantime we are getting questions from "non managed service" customers - asking us to help them. It would of helped us, to have a crystal clear timeline with buildnr and dates. This way I just need to assume "the first possible"/"worst case" that the timer will be ticking starting 14. October. This assumption can lead to us making unnecessary efforts and, for example, having to introduce another mail relay tool to bridge the gap. I do realize that I can also "pause". But 90 days per YEAR are used fast. And if the “non-managed” customer later forgets to call us for CU Updates, their company will come to a standstill because mail relay no longer works and I will not be able pause anymore until we can help then patch.