Do you have encrypt if possible selected in settings?

Thanks, this is a good suggestion, however, I checked, and "Automatically enable encryption when possible" was already disabled.

Out of curiosity, just now I went ahead and enabled "Automatically enable encryption when possible" which then allowed me to also enable "Automatically disable encryption when recipients change and encryption is no longer possible." In theory this might help disable the encryption, because after all, encryption to the recipient in this case is not possible because Thunderbird does not have a PGP key for this recipient. However, no joy, upon testing with these new settings, Thunderbird is still encrypting the message, somehow.

Also, it should be noted, although the message is encrypted, it can't be decrypted, presumably because there was no valid key against which to encrypted it in the first place. Clicking the "Decrypt" button has no effect on the displayed message.

In the Template that I am using, the "encrypt" button is not selected </blockquote>

I'm sorry, I must correct my original description here. It turns out that I was confused by Mozilla's awful user interface design which misled me into believing that opening the template would actually open the template, but instead it turns out that opening the template actually opens a portal to some alternate dimension in which opening the template actually instantiates a new e-mail object from the template...

It turns out that when I choose the menu option for "Edit Template" I then discover that the template has the "Encrypt" button selected, and the "Encrypt" button can not be deselected. Well, it can be deselected, but saving the template unfortunately does not save the deselected state of the "Encrypt" button.  :-(

So, it's a bug that doesn't seem like it will have any workaround.  :-( I did however file a bug report so there is a chance it might get fixed eventually https://bugzilla.mozilla.org/show_bug.cgi?id=2015226