Also, unless forced to plain text, every HTML formatted message from Groups.io (and that's the default format) has a "Reply to Sender" link as part of the footer that, when clicked, should fire up your e-mail client's compose window with the sender's address prepopulated as the "To:" address and the Subject line prepopulated with the topic as well.
It's easier to use that than to bother reviewing the message header information.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall