Unfortunately, they aren't usually using anything so obvious as xxx in the headers for the ones I'm getting. It's usually "Hi" or "What have you been doing lately?" ...you know, just the sort of thing that your friends write to you about.

Except that my friends don't have addresses like h123gsh91@whatever.com...

I think that's also why I always try to write mail with interesting, descriptive headers. Not just "Hi..."