Thank you for contacting Aspose Support team.
Can you please share your sample file with us so that we can investigate it further for assisting you?
Thank you for sharing the MSG file.
We have analyzed your sample code and the MSG file as well and would like to share with you that it is an expected behavior. The MailMessage API is simple mail implementation that does not message properties for the loaded message file. These lack then when the message is obtained in MapiMessage using MapiMessage.FromMailMessage. Contrary to MailMessage, the MapiMessage object loads complete message properties and thus provide the information against the desired property. Please feel free to share if you have further concern in this regard.
The MailMessage API is used for communication protocols in addition to reading such files from disc and the additional properties information is not populated once the file is read from disc. Thus, it is not possible for MapiMessage to get this information when MapiMessage.FromMailMessage is used. If you want to have the information of Mapi Properties, please use the MapiMessage to load such MSG files.