If you are in fact looking at these inappropriate emails and she is trying to be sneaking around you, then you really ought to confront this situation. She could be celebrating her anniversary with you and it is not impossible that she's having an affair on the side or at least an emotional attachment to him.
You can either try to get more proof such as by a PI or you really have enough (one email of that nature or text) should be enough to ask her the question you want to know- is something else going on. Of course, she can deny (then you can not trust her blindly because the same behavior on her part most likely will continue)
Regardless of the reason for her behavior, you find it offensive and suspicious, you want to know why and you want it to halt. It is not respectful on her part towards you or professional to carry on like that with her boss.
It is possible that you're jealous. It may also be a sense of invasion of your emotional/personal space that you share with your wife and your teen. If their relationship/interaction is fully platonic, then you've got to figure out which aspect of it is most bothersome to you. It is possible for a male and female to just be friends (and there is always limits of course. If you feel that the friendship is inappropriate in any way or consumes too much of her own time, then you can ask her to limit her interaction. The fact that he is not calling/communicating as much with you is also a bit odd but not every person is friends with their spouse's friends)
If it relates to trust issues (if you suspect even 1% that your wife is doing or feeling something other than keeping a platonic friendship, that too will be a factor in how you feel/react to this)
It is possible that you're wondering what is it that he provides that you don't and that is never a good feeling.