Today when I attempted to send mail with the web interface, I got a message telling me that my gmail address (and subsequently many client addresses) were rejected.

The error message was a popup with the "critical" icon. It says:
Message not sent. One or more addresses were not accepted.
Rejected address:

I can send mail from thunderbird ok. Occasionally replying to some e-mails will send. I cannot send anything new without getting this error message.