how to validate an email id which does not accept CAPITAL LETTER s.

you can use something like this...

function validateEmail(emailField){
        var reg = /^([a-z0-9_\-\.]+)@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/;

        if (reg.test(emailField.value) == false)
        {
            alert('Invalid Email Address');
            return false;
        }

        return true;

}

if you need to omit the capital letters then just remove A-Z from your expression, and you need to allow both the letters capital as well as lower then use some thing like this, A-Za-z like above expression is modified to allow only lower letters, if we need to allow only capital then the expression is something like this...

Only Capital

^([A-Z0-9_\-\.]+)@[A-Z0-9-]+(\.[A-Z0-9-]+)*(\.[A-Z]{2,3})$

Both

^([A-Za-z0-9_\-\.]+)@[A-Za-z0-9-]+(\.[A-Za-z0-9-]+)*(\.[A-Za-z]{2,3})$