ASP.NET HyperLink Control: mailto link

<asp:HyperLink ID="EmailLink" text='<%# Eval("UserEmail") %>' NavigateUrl='<%# Eval("UserEmail","mailto:{0}")%>' runat="server" target="_blank" />

With a "Subject"

<asp:HyperLink ID="EmailLink" text='<%# Eval("UserEmail") %>' NavigateUrl='<%# Eval("UserEmail","mailto:{0}?Subject=YourQuestion")%>' runat="server" target="_blank" />