Email form error

Hello
I have an email contact form in a VB.NET file. If I user submits the form without completing the email field, there should be an on screen error message. This is what I have in that contact form:
<span class="textStyle">Email&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><asp:TextBox ID="email" runat="server" CssClass="formStyling" Height="25px" Width="270px"></asp:TextBox><br /><br /> <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="email" ErrorMessage="Please enter a valid email address" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" CssClass="formStyling1"> </asp:RegularExpressionValidator> ...