Try this
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
ErrorMessage="Invalid Email addresss" ControlToValidate="TextBox1"
ValidationExpression="^[a-zA-Z][\w\.-]*[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$" />
Using javascript
<script type="text/javascript">
function checkEmail(email) {
if (/^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,3})+$/.test(email.value)) {
return (true)
}
alert("Invalid E-mail Address! Please re-enter.")
return (false)
}
</script>
<asp:TextBox ID="TextBox2" runat="server" onblur="checkEmail(this)"/>