I tend to agree with your T that behind sending these emails could be wanting to be in his mind between sessions. The explanation you give yourself (I just wanted to give him a heads up about a certain topic and wanted to be polite by telling him in advance so he wouldn’t be caught off guard.) might make sense to you consciously but I personally don't buy it. It sounds like an explanation fabricated by your conscious mind to get what you need but because what you need feels shameful or embarrassing to you then another, less shameful reason has to be created. I absolutely don't mean to imply that it is happening consciously or manipulatively. Also, I do not think there is anything shameful or wrong in it.

But really, where comes this idea that you somehow have to worry about him not getting caught off guard by something you tell him in session? I mean, the whole therapy is built up this way that people come in and tell what's on their mind and in that sense, it's really part of the job to get caught off guard all the time. Why do you think you need to take care of him in such a way?