Solution: Go to Settings. Click your name at the top to access your Apple Account. Look for a notice at the top of your Apple Account page that asks you to accept the new terms and conditions for Mail. Accept the new terms.
Context: Apple has introduced new AI features with Mail that requires the terms be accepted and it is causing all sorts of issues with the Mail app until you accept the new terms. I was having issues with Search, Sending mail, and indexing had also halted. Once I accepted these terms, everything worked perfectly again.
One more thing: Go to Settings > iCloud. Check the iCloud backup status. Mine had been halted, so it wasn't backing up. You may need to click Back Up Now after you've accepted the terms.
My usage situation: iPhone, Mac Mail user, iCloud account, Mac email account only. The only issue I had was on the iPhone Mail app (Mail was working fine on my Mac desktop and laptops). If this isn't your situation, then your mail issue may be different to mine.
Hope this helps someone!