Hello I have tried with this script :
https://gist.github.com/noahwilliamsson/08500bb6417015b7d6023e0b42cac34e#file-openhaystack-fix-compat-sh

Here the return :
`macOS version (major, minor): 12.6
Mail.app plugin compatibility UUID: A4B49485-0377-4FAB-8D8E-E3B8018CFC21

Mai bundle claims to support the following plugin compatibility UUIDs:
Array {
25288CEF-7D9B-49A8-BE6B-E41DA6277CF3
224E7F96-2099-499C-A501-63FB68C79CD2
D985F0E4-3BBC-4B95-BBA1-12056AC4A531
6FF8B077-81FA-45A4-BD57-17CDE79F13A5
A4B49485-0377-4FAB-8D8E-E3B8018CFC21
}

OK, /Users/moutarde/Library/Mail/Bundles/OpenHaystackMail.mailbundle/Contents/Info.plist already claims to be compatible with A4B49485-0377-4FAB-8D8E-E3B8018CFC21

Using sudo spctl to whitelist the mailbundle in GateKeeper (you might be asked to enter your login password)...

• sudo spctl --add --label 'Allow OpenHaystack Mail.app plugin to load' /Users/moutarde/Library/Mail/Bundles/OpenHaystackMail.mailbundle
• set +x

Finally, open Mail.app and goto Preferences > General > Manage plugins and check the OpenHaystackMail.mailbundle plugin.`

But always same problem mail crash when opened....

Hi.

I was having same troubles.
At the begging I was in macOS 12.2 and mail V15.
I have updated to MacOS 12.6 and mail V16.

First in Xcode in build settings, I have changed the destination version from macOS 11 to 12.3 for open haystack et mail bundle before build the app.
Same issue..
I have mail set in email app, it was my email used for icloud.
So after searching, I remember my Mac wasn't connected to iCloud.
So the trick for me was to connect my Mac to iCloud in settings.
The email used for my iCloud is a bbox.fr and work like a charm

Then everything is working now.

Thanks a lot ! I log my business account into icloud and it work now.