Ok
First, you should use regular expresions to do this
Second, you must do the same validation on the server as well bia php if you don't want hackers messing around with your data, also with regular expresions.

Code:
//javascript code
function MailTest(){
if(document.getElementById("formMail").value.toString().match("^[a-zA-Z0-9]+([\.]?([a-zA-Z0-9_-])+)*@[a-zA-Z0-9]+([\.-]+[a-zA-Z0-9]+)*\.[a-zA-Z]{2,}$")){
...