Hi,
TexBoxMode Email is supported for ASP.Net 4.5 Framework and higher. It won't work for .Net 4.0 in Visual Studio 2010.
.Net 4.5
<asp:TextBox ID="txtEmail" runat="server" TextMode = "Email" CssClass="form-control" placeholder="Enter Email" />
.Net 4.0 or lower
<asp:TextBox ID="txtEmail" runat="server"/>
Then in Page Load
protected void Page_Load(object sender, EventArgs e)
{
txtEmail.Attributes["type"] = "email";
}