Sorry to duplicate, but the reply I got to my first question didn't help, and there didn't seem to be any follow up. So I am trying to be more specific. I can receive but not send emails. I have read the page of suggested Mozilla solutions (Cannot send messages), tried them all, to no avail. I have contacted Network Solutions. Everything checks out their end. Disabling my anti-virus software does not help. I can send messages from a cell phone. As an experiment, I downloaded Opera, and tried it. I can send messages from Opera. But I don't want to use Opera. I want to use Thunderbird. The exact error message I get when trying to send is... "The message could not be sent using SMTP server XXXXX for an unknown reason. Please verify that your SMTP server settings are correct and try again, or contact your network administrator." I'd be very grateful for help with this.