Prevent Comma Separated Email Addresses in ASP.Net TextBox

Use ASP.Net AJAX FilteredTextBoxExtender

<asp:TextBox ID="TextBox6" runat="server"></asp:TextBox>
<cc1:FilteredTextBoxExtender ID="FilteredTextBoxExtender5" runat="server" FilterType="Numbers, LowercaseLetters, Custom"
    ValidChars=".@" TargetControlID="TextBox6" />