function checkEmail(emField){ //reference to email field passed as argument

var fieldValue = emField.value // store field's entire value in variable

// Begin Valid Email Address Tests

//if field is not empty
if(fieldValue != ""){
var atSymbol = 0

//loop through field value string
for(var a = 0; a < fieldValue.length; a++){

//look for @ symbol and for each @ found, increment atSymbol variable by 1
if(fieldValue.charAt(a) == "@"){
atSymbol++
}

}

// if more than 1 @ symbol exists
if(atSymbol > 1){
// then cancel and don't submit form
alert("Tolong masukkan alamat email yang benar")
return false
}

// if 1 @ symbol was found, and it is not the 1st character in string
if(atSymbol == 1 && fieldValue.charAt(0) != "@"){
//look for period at 2nd character after @ symbol
var period = fieldValue.indexOf(".",fieldValue.indexOf("@")+2)

// "." immediately following 1st "." ?
var twoPeriods = (fieldValue.charAt((period+1)) == ".") ? true : false

//if period was not found OR 2 periods together OR field contains less than 5 characters OR period is in last position
if(period == -1 || twoPeriods || fieldValue.length < period + 2 || fieldValue.charAt(fieldValue.length-1)=="."){
// then cancel and don't submit form
alert("Tolong masukkan alamat email yang benar")
return false
}

}
// no @ symbol exists or it is in position 0 (the first character of the field)
else{
// then cancel and don't submit form
alert("Tolong masukkan alamat email yang benar")
return false
}
}
// if field is empty
else{
// then cancel and don't submit form
alert("Tolong masukkan alamat email yang benar")
return false
}

//all tests passed, submit form
//alert("VALID EMAIL ADDRESS!")
return true
}

// The below function is needed only when using an image to invoke the function
//NOTE: the form in the example is named, "imageForm" and the field is named, "email". Either name your form and field the same or change the references in the script below to reflect your form and field names.

function submitForm(){
var myForm = document.form1
var eField = myForm.emailpengirim
if(checkEmail(eField) != false){
myForm.submit()
}

}