Try changing your mailto HyperLink from this:
<asp:HyperLink runat="server"
ID="lnkEmail"
ImageUrl="~/Images/emailicon.png"
NavigateUrl='mailto: <%# DataBinder.Eval(Container.DataItem, "emailaddress" %>'></asp:HyperLink>
to this
<asp:HyperLink runat="server"
ID="lnkEmail"
ImageUrl="~/Images/emailicon.png"
NavigateUrl='<%# "mailto:" + Eval("emailaddress") %>'
Text='<%# Eval("emailaddress") %>'></asp:HyperLink>
as user256103 mentioned you were missing a closing paren in your DataBinder.Eval. I think the larger issue however is the value of NavigateUrl. ASP.NET's data binding scriplet (<%# %>) allows strings to be concatenated. You'll also need a Text value for the HyperLink server control to work.