Does this happen with all your senders?

Did someone have that address in their reply-to address header?

Do you have that address in account settings - copies & folders - CC or BC these messages?

Thanks for the reply. The issue remains unresolved. I deleted IBM Trusteer from my Thunderbird address book. It only happens when I reply to one sender. It was not in anyone's header. That address is not in account settings - copies & folders - CC or BC these messages.

Is Trusteer Rapport and/or anti-virus software installed on your computer?

Hi Scooter. I had Rapport on my machine a long time ago. It was offered by all the banks. But it had endless problems. My computer is W10 home, and only uses the built in Windows firewall and anti-virus - they are up to date as is W10. I have run Malwarebytes several times and it finds nothing. I guess there is nothing in my emails that are critical. Perhaps I could ask IBM Trusteer if they like my emails! To do so you need to create an account and spend months troubleshooting with them as I have done in the past.

Is a message sent to IBM Trusteer Support when you reply to that person from Web mail?

I do not know. I would have to talk to IBM Trusteer Support which if you go to their web site you see it is hard to do. I could try a direct mailing to them. I think that would start endless exchanges with no results. If they are receiving email from me, they are obviously ignoring it. The reciprient of the email is not tech savy and simply says they know nothing about IBM Trusteer Rapport. I could re-install Thunderbird trying something like Rev Uninstaller to clean up the registry. Then I would have to somehow save my address book, etc. Since i am only dealing with a couple of emails, I think I can live with them ease dropping. I will watch for further signs of this action.

"Web mail" = e-mail account on the website of your e-mail service provider.

I am suggesting that you reply to an e-mail message from that certain correspondent by using your e-mail account on the Web. Then see if a message is sent to Trusteer.

That will tell us if the cause of the issue is on the server or in Thunderbird. I believe that it must be on the server.

I appreciate your help, but I see no way that Microsoft Outlook on the web would be sending my email to IBM. I have gone to Microsoft Live and sent an email. However Thunderbird is set to download my email, so I cannot return email from the website. I learned a lot about how Microsoft Outlook on the web works, so thanks for the suggestion. Outlook has found my Thunderbird address book and so must be somewhat synced. I was just looking for an easy solution and also to alert the community that Trusteer Rapport may be some sort of Trojan.