If you create custom document libraries which are not of the type=101 then you cannot email enable these document libraries. We did some research with reflector to find out why:
When SharePoint checks if a template can be email enabled it checks "SPList.ServerTemplateCanReceiveEmail". If we look at the code of that function we can see that email enabled lists are not supported on Meeting Workspaces and that the templateType is checked trough "SPEmailHandler.HasHandler(templateType)"
If we look at the code of "HasHandler" we can see that only 4 listtemplates can be email enabled : Announcements, Events, Document Libraries and Posts (blog).