This is not a reply but an amplification of the above statement. Since upgrading to Big Sur, the following have happened to my email: Emails that I want and have, up until now, been in my inbox, end up in junk; moving these back to inbox sometimes works but not always; clumps of deleted emails return sometimes almost immediately, often after an hour or two.
The impact is that I have to watch both my junk box and my inbox, spend unnecessary time transferring emails back to the inbox and redeleting emails.
I have talked to two Apple support persons about this who have researched the problem without success. One suggested that I worked my email within Gmail, not Apple's mail. While Gmail has its advantages, its "From" does not work like Apple's "From", and I find many of its doodads complicating rather than easing working with email.
I rarely work with this Community network because much as I like the idea, do not find it easy to implement the suggestions since they are, so often, couched in technical language that I do not understand. I have sent my issues to Google but not had any reply.
One suggestion I have picked up is to remove and replace my email account, for which thank you. I am posting my experiences in the hope that others may have some solutions that I might understand - thank you, in advance, for those, too.