That happens automatically on the server, regardless of Thunderbird settings. I doubt there is an option to disable that behaviour, but give Gmail settings a look, maybe there's something in there you could turn off; it's been ages since I last opened Gmail settings.

Thanks. That doesn't solve my problem, but it's a big help as I couldn't figure out if it was a Google/Gmail issue or a Thunderbird issue. I will re-focus on Gmail. I could not find any Gmail setting that would turn this on and off; I'll search some more. Thanks again.

Stans is correct. This did not happen in the past.

Google/gmail decided it was a good idea to put emails you send via it's smtp server into the server/webmail account 'Sent Mail' folder. Reason: if Pop user was accessing and using webmail account for any reason, then they would have access to seeing what emails they sent from a device using POP.

The downside is a pop user does not know this is occuring until they access webmail. The emails in the 'Sent Mail' folder/label are also in the 'All Mail' and they use up quota.

It is possible to create a filter in the webmail account. You could try... If 'From' is your email address then you must have sent email, so delete. Enter your email address and click on 'create filter' and choose the 'Delete it' option.

It worked! Created a filter in the Sent folder in the Gmail webmail and my message went to its recipient, showed up in my desktop Sent folder, and went straight to trash on the webmail page. Of course, that means it is still being saved in Google-land, but the trash folder automatically deletes anything >30 days old, and I can live with that.

Thanks to both of you for figuring this out.