Please post a screenshot of the error. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Hello! Attached it is the notice that keeps popping up in Thunderbird. The error message that we receive when we try to send an email states: "Sending of message failed. The message could not be sent using SMTP server mail.eaglechiropractic.ca for an unknown reason. Please verify that your SMTP server settings are correct and try again, or contact your network administrator" This has started to happen today and no one on our end has changed any settings. Everything was working with no problem yesterday. PS. I just realized that these questions are public based on your previous edit?? All I wanted was to speak to someone in private and I can't find a number to call for technical support.. Any chance this can be moved to a private conversation?

If I go to view the certificate, this is what you will see

Your email or hosting provider issued a new cert on 2/5/18. I bet this is when the trouble started. The cert has been issued for *.web-hosting.com. However, your email or hosting provider's server is mail.eaglechiropractic.ca. There's a mismatch, and hence you get the Security Exception prompt.

Your options are:

1. You can confirm the Security Exception. The cert looks legitimate to me. But ultimately it is up to you to decide whether you trust it or not.

1. You can complain to your hosting provider about the problem and wait until they'll fix it. Don't hold your breath though.

Any chance this can be moved to a private conversation?

This forum is the official Thunderbird support venue. There is no phone support for Thunderbird. It is of course up to you to hire a professional to help you with your problem.