I don't know if I understood well your question but you could try something like:
//Check if the active inspector is a MailItem.
if(App.ActiveInspector().CurrentItem is OutlookApi.MailItem){
//do something.
}
Or you can use the event ItemLoad
private void OnItemLoad(OutlookApi.COMObject item){
{
if (item is OutlookApi.MailItem){
//do something
}
}
I hope that this can help you.
Regards