Javascript: Email Validation function

function ValidateEmail(str)
{
var at="@"
var dot="."
var lat=str.indexOf(at)
var lstr=str.length
var ldot=str.indexOf(dot)
if (str.indexOf(at)==-1)
{
alert("The eMail address '@' convention appears to be invalid.")
return false
}

if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr)
{
alert("The eMail address '@' convention appears to be invalid.")
return false
}

if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}

if (str.indexOf(at,(lat+1))!=-1)
{
alert("The eMail address '@' convention appears to be invalid.")
return false
}

if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot)
{
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}

if (str.indexOf(dot,(lat+2))==-1)
{
alert("The eMail address 'dot' convention appears to be invalid.")
return false
}

if (str.indexOf(" ")!=-1)
{
alert("The eMail address spacing convention appears to be invalid.")
return false
}
return true
}