Mail stopped working under 11.2

have you tried deleting an account and re-add it to see if it comes back?

Sometimes prefs and such can get borked during updates for what ever reason

(phase of the moon, neutrinos, etc.). Over the years I have had that happen

but usually for just one account.

FWIW, Mail has been working fine for me on my M1 MacBook Air even after

just updating to 11.2.1.

Also, what shows up in the Mail Connection Doctor window for your accounts?

FWIW, had an issue with iCloud mail recently where I could

receive emails but there was no option to send emails using

the iCloud account. Disabling and then re-enabling fixed it.

Mail has become somewhat flakey over the years but most of the

stuff I have seen is minor nuisances.

Glad it was sorted out.

Personally, I haven't seen a lot of real issues with Big Sur

with the exception of external boot on my M1 MacBook Air

which is more an M1 issue I think than a Big Sur issue.

All of the "annoyances and quirkiness" I have seen have been with

Mail. One I still haven't figured out is why sometimes quitting it

is normal and other times it spins its wheels for up to a minute or

two before closing. There have been a few other inconsistent

"oddities" with it as well.