How do i check for certain words like "hotmail" "gmail" "yahoo" within an email field using Javascript?

I have a form validation function working on my form but it currently only checks that "@." are included the code so far..

Code:
// test if valid email address, must have @ and .
var checkEmail = "@.";
var checkStr = theForm.email.value;
var EmailValid = false;
var EmailAt = false;
var EmailPeriod = false;
for (i = 0;  i < checkStr.length;  i++)
{
ch = checkStr.charAt(i);
for (j = 0;  j < checkEmail.length;  j++)
{
if (ch == checkEmail.charAt(j) && ch == "@")
EmailAt = true;
if (ch == checkEmail.charAt(j) && ch == ".")
EmailPeriod = true;
	  if (EmailAt && EmailPeriod)
		break;
	  if (j == checkEmail.length)
		break;
	}
	// if both the @ and . were in the string
if (EmailAt && EmailPeriod)
{
		EmailValid = true;
		break;
	}
}
if (!EmailValid)
{
alert("Please enter a valid email address");
theForm.email.focus();
return (false);
}