Today we are announcing the availability of quarterly Cumulative Updates (CUs) for Exchange Server 2016 and Exchange Server 2019. These CUs include fixes for customer reported issues, all previously ...
Updated Jun 29, 2021
Version 1.0
Blog Post
I have also had enormous problems with Outlook client connectivity since installing Exchange 2016 CU21 last week. I have a ticket open with Microsoft, but no resolution yet.
The problem is definitely related to AMSI. Disabling AMSI by remarking out the configuration in web.config did not help.
We use Sophos, but this may apply to other AV providers. Sophos advised disabling AMSI on the server agent for testing purposes. I did this directly on the server agent with a support engineer. No help.
Today I created and applied a profile from Sophos Central disabling AMSI on the server. As soon as the profile applied Outlook connection speeds returned to normal.
I do not know if this is a Microsoft issue or a Sophos issue. I believe it may be a Microsoft issue, as I've read that other admins using other AV products have had similar problems. Regardless, I do hope it's resolved soon as this is a potentially important security issue.
Also, I noted today that a new security update for CU21 was released (Released: July 2021 Exchange Server Security Updates - Microsoft Tech Community). No word in the article if this AMSI issue is addressed by this update, and the KB link in the article is not correct.