I just borrowed a quadratic equation from the MathML Wikipedia site and grafted it into an HTML document. It imaged fine in Safari 7. It should be noted that neither the latest Google Chrome or FIrefox rendered the same content accurately.
Laying out your entire email message (including MathML) in HTML can be tedious. Once it is displayed in the browser, each use of the command+I will put the browser content in a new Mail compose window. So, you cannot add an equation to existing Mail composition. Select, copy and paste (or paste and match style) will not work either.
Depending on the importance you place on email equations, at one time, Design Science (MathType) claimed compatibility with Apple Mail (4.5) where you could input your equation in MathType and it could then be placed within the email. The mail recipient would not need a copy of MathType installed for this to work. MathType even allows you to key your equation in with LaTeX syntax to generate the equation.
You could check with them about functionality (or restrictions) with Mail 6 and 7 on OS X, as it might provide a more productive way to achieve your goals.