I was going to give a list of possibilities for 気持ち悪い originally, but I thought that "creepy" would probably best suit an e-mail situation (unless there were graphic and disturbing pictures in it... then "disgusting" would have been more appropriate). At any rate, it's notoriously a difficult word to translate, but the point is that the person saying it feels that something is disagreeable in some way to them. Here are the options given at the Kenkyusha Japanese-English dictionary:
気持ちが悪い 《事が主語》 be unpleasant; be disagreeable; 《人が主語》 feel ill [unwell, funny] (気分がすぐれない); feel sick (吐き気がする)
According to that, if a thing is the subject, which it happens to be in this case (e-mail), the glosses are "unpleasant" or "disagreeable". The others are for when a person is the subject. "Creepy" may be a bit strong, but it's somewhere in that neighborhood.