Email Validation

//To Check for proper email address
if(crmForm.all.emailaddress1!=null && crmForm.all.emailaddress1.DataValue!=null)
{
                    var isval = validateEmail(crmForm.all.emailaddress1.DataValue);
                     if(isval==false){
                     event.returnValue = false;
                     crmForm.all.emailaddress1.DataValue = null;
return false;}
}
if(crmForm.all.emailaddress2!=null && crmForm.all.emailaddress2.DataValue!=null)
{
                     isval = validateEmail(crmForm.all.emailaddress2.DataValue);
                     if(isval==false){
                     event.returnValue = false;
                     crmForm.all.emailaddress2.DataValue = null;
return false;}

}

if(crmForm.all.emailaddress3!=null && crmForm.all.emailaddress3.DataValue!=null)
{
                   isval =  validateEmail(crmForm.all.emailaddress3.DataValue);
                     if(isval==false){
                     event.returnValue = false;
                     crmForm.all.emailaddress3.DataValue = null;
return false;}

}

function validateEmail(mailstring)
{
     var searchStr='@';
     var isEmail = mailstring.indexOf(searchStr);
     var searchStr2 = '.';
     var isValid = mailstring.lastIndexOf(searchStr2);
if(isEmail == '-1' || isValid == '-1' || (isValid < isEmail) || (isValid == (mailstring.length-1)) || (isValid > (mailstring.length-3)))
{
    alert('Please enter proper email address');
     return false;
}


}