After installing Big Sur Public Beta 4, Mail was stuck perpetually at "Mail Upgrade".
Fixed the problem by going to folder ~Library/Mail and renaming the directory for the mailboxes from "V7", which was Catalina's version to "V8" which apparently was what Big Sur mail was looking for. Apparently the rename didn't occur during the Upgrade. Restarted Mail and it then completed the Upgrade, went to Optimizing, and Mail loaded and is functioning properly.
Fixed the problem by going to folder ~Library/Mail and renaming the directory for the mailboxes from "V7", which was Catalina's version to "V8" which apparently was what Big Sur mail was looking for. Apparently the rename didn't occur during the Upgrade. Restarted Mail and it then completed the Upgrade, went to Optimizing, and Mail loaded and is functioning properly.