Rated 2 out of 5 stars
I'm giving this more than one star because it has potential.
Nevertheless, I'm afraid I have to reiterate what previous commenters have said: it appears in the UI but doesn't appear to actually use the selected SMTP server.
I'm using Thunderbird/126.96.36.199 (20081209) without any other add-ons installed.
Additional functionality that would be nice: selecting a default SMTP server based upon WAN IP, or making it switch to another if the first fails.
Double-six: I'm very interested to see your improved code -- and contribute to improving it further.