2024 UPDATE > Charter support is pretty clueless regarding apple mail, they don't care.
Apple support doesn't really know or want to know anything about charter, no surprise there.
If you don't have the technical patience to work around/ through this issue, you may as well give up and use a new email account other than charter.... the issue comes & goes and then comes back & goes again!
And of course charter has webmail system everyone can use directly rather then inside apple mail.
The problem is charter webmail & apple mail don't work well together and likely never will. After batting this around off & on for about 5 years, I notice it's especially disfunctional (or temporarily broken) right after charter does a periodic system update, but nothing bad really ever happens when apple updates its system. My conclusion is its clearly a charter issue, which causes some kind of recognition/ communication glitch that seems to last awhile.
I've solved the problem most quickly and the same way every time it happens by simply deleting any charter email accounts from apple mail (on every apple device - imac, ipad, iphone), then restarting the apple devices and setting up all the same charter email accounts in apple mail again, always as imap accounts and using incoming/outgoing server designations <mobile.charter.net>. I also suspect that since charter still has options for both pop & imap accounts (for users that don't know why they want them) this creates another possible technical glitch that may contribute to this issue although I don't understand why.
Anyway.... my final answer is: delete charter email accounts from apple mail, restart. set same accounts back up, restart.... may have to repeat process a few times until it works.... its stubborn, but normally gives up & works again.
