Just a thought. What about you changing the SQL to get this email address in the format you want? Then you can create this like,
<asp:HyperLink ID="Hp1" NavigateUrl='mailto:<%#Eval("mailto") %>' runat="server"><%#Eval("mail") %>
SELECT email as 'mail, 'mailto:' + email as 'mailto' from MyTable
Hope this helps :)