...and no real warning. Different than other upgrades.

IDK how to fix it, so I've been recreating all my accounts in Mail.

It was in an Apple users forum. "I deleted my anti-virus (Avast), re-booted and iCloud mail returned to normal. Or you can turn off the AV function in System Prefs > Network."
I turned of the AV function. It worked like a charm. So far so good. Going 24 hours without email is just too much.

PS Turning off AV function didn't work too well-it kept turning itself back on. So I deleted a the Avast program.

a fee. if yes , pay the fee and argue mostly later.

if they are reluctant , stop everything and ask for their name and a real supervisor.

2. i would tend to think that they have backups if the mail is iCloud.

3. in your home directory in the Library foldee , the Mail folder lives there. you might need the GO menu. make a duplicate of the Mail folder, Preferences and Keychain folders

Signed

Retired Mac Computer Consultant. Experience dating back to Apple I through OSX 10.11.6 EL Capitan then retired.