Using version 5.6 fixed my issue. Thank you.
I am having a similar problem with version 5.8. Here's the error:
System.ArgumentNullException: Buffer cannot be null. Parameter name: buffer
at System.IO.MemoryStream..ctor(Byte buffer, Boolean writable)
at Aspose.Network.Outlook.MapiMessage.Save(Stream stream)
This doesn't happen in 5.6. I also tested 5.9 and the error is the same.