Problem : FollowUpManager.GetCategories(mapi) return an empty array.
I think I need to specify extended properties when fetching the email (FetchMessage have an overload that takes extended properties), but didn't found anything in your documentation.
MapiMessage categories can be obtained using FollowUpManager which contains functions like GetCategories(), AddCategory(), RemoveCategory() and ClearCategory.
Following is a sample which demonstrate these features and if we have MailMessage then that need to be converted to MapiMessage. We can also observe that once the category is assigned, that can be obtained using GetCategories() function. Please give it a try and let us know your feedback.
I have tried this scenario and observed that category is not maintained in MailMessage while fetching message from exchange server. Please spare us little time as we are discussing this issue and will write back here as soon as some feedback is available in this regard.
I was able to reproduce this issue at my end using the latest version of Aspose.Email for .NET 4.7.0 and have logged it as enhancement ticket under id: NETWORKNET-34548 in our issue tracking system for further investigation by our development team. Once there is any information available in this regard, I'll update you here via this thread.