I have a very close friend of mine from college and we trained together at the same dojo with a private instructor for a couple of years. I graduated a year earlier than he did but now every time I get an email from him it ends with "Sensei" down at the bottom.
He does teach classes but he's never taught me anything. Would it be disrespectful to ask him to drop the sensei thing at the bottom of his messages. I'm training in a dojo in a totally different state tahn he is and it's really starting to get under my skin.