Update 4/23/2024: We have now released April 2024 Hotfix Updates which address known issues in March 2024 SU updates.
Microsoft has released Security Updates (SUs) for vulnerabilities found in:
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Updated Apr 23, 2024
Version 8.0
Blog Post
embert2165 I would settle on The_Exchange_Team updating these release post to add in these two bugs to "Known Issues" so people installing this will have some warning.
* Causes "Unread" Icon Envelope to appear on certain messages received after applying March 2024 SU
* Causes FAST Search with Outlook Desktop Clients to not work and unable to communicate with server.
Like you said it's been almost a day now since these bugs were reported.
I noticed both bugs only apply to users that restarted Outlook clients AFTER March 2024 SU is applied. Search worked yesterday afternoon for users I had that had not closed their outlook clients all day. And the "Unread Envelope Icon" only appears on emails FROM users that have restarted their Outlook clients since the update and sent an email to another user within our Domain. Emails from either Outside or Anonymously authenticated (Such as a bunch of automated alert systems I have), and users sending email from OWA or ActiveSync clients are not affected. Ex I can send myself an email from my Outlook Desktop client and it has the icon. If I use my phone's ActiveSync mail app, no icon.
My guess it's some setting Outlook clients are misreading from either Autodiscover or SOMETHING from the Exchange servers with the SU applied on startup causing both issues.