They don't. I believe using this setting it is a good way to prevent tracking by pixel. In addition, I think enforcing plain text emails would disable any clie

They don't. I believe using this setting it is a good way to prevent tracking by pixel. In addition, I think enforcing plain text emails would disable any client side tracking events. EDIT: Well it looks like that setting in Thundebird prevents any remote connections. Not just pixels, so that be good enough. However emails often have a text back up that is going to be better formatted then a hampered html email. EDIT: To prefer plain text in Thunderbird it is View -> Message Body As -> Plain Text