A nice start, and good to see you keeping the class simple and to the point. The actual number of lines could be whittled down with some rewriting (look where you're repeating yourself).

I'm not a fan of the whole 'reset' idea. Best, in my opinion, keeping two different email messages separate from each other. I'd perhaps also throw in a static method rather than having to create at least one instance of the class with a global variable: email::instance()->to('blah...