When you send an email from your iPhone it ends up with a default signature that reads "Sent from my iPhone". Whenever I see an email with this at the bottom, I think "what a tool, they can't even change their email signature". Don't be a tool. Change your email signature. Or if you can't come up with a better signature, just get rid of it and have none.

This is pretty easy to change. From the home screen of your iPhone, tap on "Settings". From the settings, tap on "Mail, Contacts, Calendars". Scroll down to the bottom of the "Mail" section and find the entry called "Signature" and tap on it. You can then create a new signature or at least get rid of the default signature.