Thunderbird, cannot send emails out.
Error message:
"Sending of the message failed. An error occurred while sending mail. The mail server responded: <ec2-54-219-72-248.us-west-1.compute.amazonaws.com[54.219.72.248]>: Client host rejected: NO THANKS. Please check the message recipient "patrick.gilroy@harmonydesigninc.com" and try again"
Occurs regardless of recipient, whether replying to an incoming email, or composing a new email.