Today, we are announcing the availability of the 2025 H1 Cumulative Update (CU) for Exchange Server 2019 (aka CU15). This is the last CU we will release for Exchange Server 2019.
CU15 includes new ...
Updated Feb 26, 2025
Version 9.0
Blog Post
Regarding Hybrid Exchange environment:
According to rumours, MS will block some day the email flow between on-premises E2019 and EXO cloud if CU14 exists. However, the article informs that MS is supporting CU14 & CU15 .
Please confirm: When Microsoft is starting rejections of email flow between on-premises & cloud if CU15 is not installed?
Nino_Bilic
Microsoft
MicrosoftThis is not something that is on the calendar today. There is no "defined date".
Our goal is not to block email. We are not in the email blocking business. Throttling and blocking is for significantly out of date servers, and it is our way of nudging customers along to update the servers that has a connector to Exchange Online to be supported.
We take various things into consideration when deciding what should be blocked. Things like time, severity of security issues that we fixed since etc. It is a process that humans decide on (it is not some sort of "block everything that is older than a year" for example).
So - in short - I have no date to give you. Will it happen some day? Definitely. Do we plan to do it as soon as CU15 is unsupported (in October 2025)? No. Could there be some sort of high-severity security issue that makes us decide that CU15 should be throttled/blocked sooner than we plan? Theoretically, yes.
The bottom line is - always stay up to date and you do not need to worry about any of this.