Hi
If you want to use link then you can use following syntax,
<asp:hyperlink id="hlnkSendMail" runat="server" navigateurl="mailto:user@example.com?cc=user@example.com&bcc=user@example.com&subject=MessageTitle&body=Message Content" target="_blank" text="Send Mail Link" xmlns:asp="#unknown"></asp:hyperlink>
And, if you are require to use button you can use following syntax
<asp:button id="btnSendMailClient" runat="server" onclientclick="return OpenMailWindow();" text="Send Mail" xmlns:asp="#unknown" />
Add following javascript function also,
function OpenMailWindow()
{
window.open("mailto:user@example.com?cc=user@example.com&bcc=user@example.com&subject=MessageTitle&body=Message Content");
return false;
}