Hello JGWheeler,
Based on what you've told us, this appears to be happening with your iCloud account. @me.com, @mac.com, and @icloud.com are all iCloud account email addresses. The error message mentioned “p46-imap.mail.me.com”, which is a server port for your iCloud account. You can learn more about iCloud account aliases here: iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support
To translate the message a bit, Mail is saying that it couldn't open the mailbox "Chartr" on the server (on the iCloud side), because the mailbox doesn't exist.
Here are a few things to check:
Are you able to send and receive emails using your iCloud account?
You can test that by composing a simple test email and sending it directly to yourself using the same email address. You should receive the email shortly. If you do, that means your server settings are correct for the account, and the issue is most likely caused by the rule itself. So rebuilding the account or signing out and back in won't be needed to correct the issue.
Is the term "Chartr" used in a rule?
If so, disable the rule(s) and test for the issue again. You can test by forwarding an email to yourself that would typically be caught by the rule. If the issue doesn't continue, we know a rule was the culprit.
If the issue is related to the rule based on the steps above, it indicates the mailbox may be the root cause.
Check to see if the mailbox exists on iCloud.com/mail. If you don't see it, that explains the issue. The mailbox doesn't exist and iCloud cant sync with it. You'll need to create it or change the rule.
If you see the mailbox, check the spelling.
If the rule and mailbox name are different in any way, it could cause an issue. Make sure capitalization matches and that there are no leading or trailing spaces in both the mailbox and rule.
Thanks.