Hi,
It's probably Thunderbird causing this. Can you check the following setting in Thunderbird?
If you have selected: View > Message Body As > Original HTML, then the Content-Type: text/html section is displayed, but if it is not present in email then Content-Type: text/plain information is displayed.
My thought was to replace this email with welcome email, to avoid the porblem, but now I see that this workaround won't be possible.
This won't work. Once a subscriber has received the confirmation and clicked on the confirmation link he never receives the confirmation email again. It's just meant to confirm yhat you can use the email address. Whereas a subscriber can receive a welcome email for each subscription to an indidivual list.