Well, a while ago I seem to remember it being decided not to scan e-mails, but that doesn't mean no-one will. I wouldn't worry too much, just keep it low key. At the moment they are interested in terrorists and big dealers, not guys sharing small amounts of weed. Try to avoid the subject of actually paying for it at least. Also, if you can, ensure that if you do get arrested you can always claim it was someone else, don't let it be possible to prove you were involved.