"To:" mail field changed without my action

Hello All,

I'm a little worried about this. I just noticed that for many of the emails that are in my inbox in Apple mail, the "To:" field on received messages has been changed to "Holidays in Viet Nam." with my email address <between the carrots>. There's no functionality loss, but, to be best of my knowledge, I definitely did not change that setting. So the question is, who/what did. Can anyone help me track this this down? I've posted a screenshot. What controls that "to:" field and how can I deal with it? Just to be clear, this is one example: the problem seemingly has been applied retroactively (but not uniformly) for months of emails. Have I been hacked? What gives?

Justin