I have no problem with an opponent telling me what happens next, that's all part of the fun of trash talk when having a laugh with each other. Of course if my oppo asked not to know in advance, which a few have, well that's something different.
Keeping you informed of whats happening is a good thing, never nice to have a couple of weeks go by with no word.
Having a nice surprise when an oppo attaches something else to the PBEM email that you weren't expecting e.g. new sounds, skins or scenarios that they recommend is cool too.