@grannydo -- Since the error states “ESMTP server not available” this indicates a delivery failure. If you are seeing this message “from remote mail server after RCPT” then it is an issue with the recipient server. It is not currently able to receive E-mail.
To what domain-name --- the part after the "@" are you trying to send?
> It looks like the email is being sent as I get a copy in my sent email folder.
Your message has successfully been sent from your computer to Shaw's E-mail server. However, that does not indicate that Shaw's mail-server was able to send to the recipients's mail-server. For example, if you were sending to firstname.lastname@example.org, and the mail-server for "telus.net" was offline, your intended recipient would not receive your message. Mail-servers are configured to try multiple times, say once per hour for three days, to try to overcome any "temporary" outage. You may not notice that it took several hours for the recipient to see your message.
I use Thunderbird as my email program on a Surface Book running Windows 10. For years, we have downloaded and sent emails using Thunderbird. Our email address is a free net account (Vancouver Community Net). The outgoing server is mail.shaw.ca. As of mid August, when we try to send emails we get the following message:
"An error occurred while sending mail. The mail server responded: <email@example.com> sender rejected. AUP#SNDR. Please check the message and try again."
I have spent hours in a chat with Shaw, and they will not help at all. Is anyone else having this problem?