I know you said this is through Outlook, but I wanted to add to what Markart
said about Firewalls and ports.
Once I put my home built firewall (used Smoothwall), my mother had trouble sending mail from Thunderbird using her comcast e-mail. Now, prior to me doing this, she had no problems. Once Smoothwall was in place, she could get mail without any problems, but sending was an issue.
Turns out I had to open up port 587 in the Smoothwall firewall for it to work. I know in her settings in Thunderbird, she does not have a username and password stored for the SMTP server (just the POP3 server to get her mail).
So, if you have recently made a change with a router or firewall, and do not have a password set on your SMTP server, make sure that port 587 is open.
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