Microsoft has released Security Updates (SUs) for vulnerabilities found in:
Exchange Server Subscription Edition (SE)
Exchange Server 2019
Exchange Server 2016
SUs are available for the f...
Updated Aug 25, 2026
Version 3.0
Blog Post
We have 4 Exchange SE with activated mrsproxy on EWS. One from these has the latest su installed since yesterday.
since then this server has problems to handle migration requests..
Error: CommunicationErrorTransientException: The call to https://xxx.yyy/EWS/mrsproxy.svc failed. --> Access is denied. --> Access is denied.
strange...
We accidentally patched another Exchange SE server to the latest August SU; same here, it can no longer accept migration requests.
Existing migration jobs are terminating with this error.
Fehler: TooManyTransientFailureRetriesPermanentException: The job encountered too many transient failures (61) and is quitting. The most common failure is CommunicationErrorTransientException with the hit count 60.
were at the beginng from a large mass migration.It couldn't be a worse time.Finally,
The MRS service from the July update can no longer access the backend of an August server.
Microsoft has confirmed this, and updating the remaining servers should resolve the issue.
We tested this by reconfiguring the load balancer and migrating only mailboxes from patched servers to the cloud.
This worked, and we will now patch the remaining servers.- Nino_Bilic
Microsoft
This exception (TooManyTransientFailureRetriesPermanentException) looks like a service migration error (vs. your Exchange Server throwing this exception?) Please open a support ticket if you are working on migrations to Exchange Online.
The truth is - we are not aware of any migration breakage after August update is installed; that's not to say there is not a scenario that might be impacted - rather we have not received reports that were verified like a regression in August 2026 update. But support tickets could help us identify if there is such a thing...
EDIT Aug 24 2026: I think we have now connected all the dots. Acknowledging the fact that if not all servers involved in MRS migration are updated to August 2026 SU, MRS migrations can fail. Update all servers to August 2026 SU and it will be solved. Added a known issue to the blog post now.
Hello,
We are a one-server Exchange SE environment and are experiencing the issue mentioned regarding MRSProxy errors. I have tried just about everything -- including recreating the endpoint. I believe the issue is not just affecting DAGs. This issue occurred after the 8-26 SU.EDIT: Setting MRSProxy to false for the directory, renewing the exchange auth cert, and running HCW again solved my issue.
This is not our experience. Just updated 4-node DAG Exchange SE with August SU. Did multiple test migrations in both directions afterwards across multiple servers and all were successful.
