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Hello lucfrommercier,

First, make sure you have restarted your Mac to clear out any potential issue between Mail and the macOS.

Shut down or restart your Mac – Apple Support

If that does't help, let's try to narrow this down and isolate it more by testing in safe mode and a new user account.

Safe mode prevents certain software from opening and also forces the computer to run some disk checks similar to First Aid. This can actually resolve some issues. More information on starting to safe mode and steps to take after starting there can be found here: How to use safe mode on your Mac - Apple Support

Intel processor

1. "Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.

2. Release the key when you see the login window, then log in to your Mac. 

3. You may be asked to log in again. On either the first or second login window, you should see "Safe Boot" in the top right-hand corner of the window."

If the issue continues after the steps from safe mode, create a new user account (administrator) to see if the behavior happens there. This will let us know if it is system-wide or something within your user account: Change Users & Groups settings on Mac

Delete a user or group on Mac (to remove the test account once completed)

Let us know what you find.

Cheers!