If messages won;t transfer, or Mail shows other inconsistent behaviour, try rebuilding Apple Mail’s database by deleting envelope index files...
Quit from Mail, and return to Finder.
Open the Library folder (either make it visible, or hold down option key while using the "Go" menu, or use the "Go to Folder" command).
Open the "Mail" folder. Open the latest "V" folder (it is "V9" for Monterey, "V10" for Sonoma)
Open the "Mail Data" folder.
Delete all files that begin with "Envelope Index"
Optionally remove saved application state as well. Go to "Saved Application State" in "Library" folder. Find "com.apple.mail.savedState". If it's an alias, find the original and delete it, otherwise just delete it.
Launch Mail again. it will show a window "Message Import". Wait for this to complete.
I usually write a blog post about once a week. The latest post can be viewed here: Avoid Microsoft: If you don't really like computers much you could make things a bit better for yourself. (posted 2024-12-04 at 12:05:50). I do podcasts too!. You can listen to my latest podcast, here: OJB's Podcast 2024-08-22 Stirring Up Trouble: Let's just get every view out there and fairly debate them..
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