JavaScript:

<script>
<!-- Hide
function checkEmail() {
if (/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(FormName.Email.value)){
return (true)
}
alert("Invalid E-mail Address! Please re-enter.")
return (false)
}
//-->
</script>

.........................

<form name="FormName">
<input name="Email" value size="95" onBlur="return checkEmail()">
</form>

ASP:

<%
' User Name <username@domain.com>
Function IsValidEmail(strEmail)
Dim bIsValid
bIsValid = True

If Len(strEmail) < 5 Then
bIsValid = False
Else
If InStr(1, strEmail, "@", 1) < 2 Then
bIsValid = False
Else
If InStrRev(strEmail, ".") < InStr(1, strEmail, "@", 1) + 2 Then
bIsValid = False
End If
End If
End If

IsValidEmail = bIsValid
End Function
%>