Server names can be very important. Share screenshot of the SMTP server setting pane of the good one and the failing one, identifying which is which.

Here you are. The first one, mruthvenlaw.com works. The second one, mruthven.com does not work.

In your prior post, you stated that this one did work. I'm confused. Have you spoken with siteground? That may be your next step.

I haven't yet spoken to siteground about this problem. What are you confused about?

That you posted it worked in prior post and now you post that it doesn't and that, despite stating you use siteground, you have yet to pursue with them.

My first post in this thread said that one does not work. I will ask siteground, but they are not much help with Thunderbird users.

I know nothing of Siteground, but that they recommend email settings with no security is in conflict with what most all other email providers require. Normally, I would suggest a different port, SSL/TLS, etc. I wish you well. If you do get a response and update on settings, please share. And, Merry Christmas. :)

This link may help. https://www.siteground.com/tutorials/email/mozilla-thunderbird/ the settings seem different from what you are using.

Thanks. I'll try those recommended settings later. However, I'll again note that the email account that works (sending) has the same settings as the one that doesn't work.