Mail Crashes at aunch on BigSur

Start Mail while holding down shift key.

If it doesn’t crash, select a Mailbox to view. If it crashes, note the message it was trying to load, if possible.

Mail has been known to crash when loading certain messages.

Start Mail again in same way, and drag the viewing pane so it is completely closed and only the list is showing. Select that message and see if it crashes, or select a few separately. If no crash, double-click the suspect to see if it crashes when opened.

if it does crash, try deleting the message instead of opening. If you need to view the message, try using the web portal for your email account and move or delete it from there.

So, it never opens whatsoever?

Are all of your email accounts IMAP or some other server-based account (i.e. not POP).

You might try removing them all from Internet accounts, and then try to open Mail.

Another drastic step would be to delete the Mail container in ~/Library/Containers/

If you user Library is not displayed in your Home, hold down the Option key and select Library from the Go menu in Finder.

You will have to set up all of your preferences and wait for it to index all of your messages--It will say "downloading x of y messages," but it isn't really downloading. It is just scanning the already downloaded messages.