What does the keyboard type on it's own. Do you recognize what it types on it's own?
Does problem occur only while composing email? What email client?
Tried the keyboard on a different computer? If it acts up on a different computer you might consider replacing the keyboard as most of them are dirt cheap nowadays.
A woman at my company was typing an email and while she was typing, the keyboard took over and began to add text on it's own. It added atleast one whole paragraph of words and stopped. Also, when she was typing a greeting on another email, the font suddenly changed and large amounts of spacing was added between each letter. Does anyone know what could be causing this? I have removed all adware and the machine is clean. I have also updated outlook. Any suggestions?