Do you have the "send later" addon installed?

i don't remember adding it, Wayne; don't see why i would have. How would I check, and remove it if it was added by a "helpful" tech during a service visit?

Tools > add-ons

I'm not being intentionally dense, but when I go to Tools -> Add-ons, it just shows me some add-ons I could add; it doesn't say, "here are add-ons you (or someone) loaded on your machine." Does this mean I have no add-ons, or is there something further i need to type in to find this out?

If the extentions tab is empty then you do not have addons.

When you send mail, do you use the mouse to "send"? Or do you use the keyboard shift+enter?

Wayne, I'm afraid I didn't know to look under the "Extension" tab--but I don't have "Delayed send" there; just "Lightning" and "Show entire body"

Sometimes I send with touchpad/mouse, sometimes with Control-Enter.

Ctrl + Shift+Enter is send later. It is easy to rest your left hand on CTRL and not be aware of it. At least it is for me.

Alternatively, do you see any red on the two monitors in the bottom left of the Thunderbird screen?

Thanks, Matt. No red on the lower left, but I could easily have hit Shift along with Control. I'll keep an eye out for that.