Email scam technique? How do I cure it?

I will receive two emails, one from abc@xyz.com, the other from HOW@FGH.com. I click on abc in the inbox, and get the HOW message. When I look at that email, and click on HOW's display name, it will show HOW's correct email address. But if I hit Reply, the response will be sent to abc.

This has been happening for the past month or two, only on occasion. It began by crossing messages from HostGator with personal friends. But just today, it crossed two individuals, both of whom I have had contact with in the past, but neither of whom are in my address book. I have Mail version 11.1 (3445.4.7), and High Sierra 10.13.1, although I think this problem started before I most recently upgraded. Any one have any thoughts about what is happening here, and how to cure it?