Hi,
you should move the Template-resource into the Application.Resources (either directly or via a merged ResourceDictionary). Then you can instantiate it by
DataTemplate mytemplate = Application.Current.Resources["EmailItemTemplate"] as DataTemplate;
mt[0].ItemTemplate = mytemplate;
I'm not quite sure if that works with the resources "Name" property; with the "Key" property it will and I've only used this with Style-resources but I think other resources will work as well.
Cheers.