I am sorry for the delayed response on this.
We had completed the investigation much earlier but forgot to post the reply. The sample message shared by you does not have an in-line attachment. I have verified that using my below source code. Also, I am able to extract the attachment using the MapiMessage Class.
MapiMessage msg = MapiMessage.FromFile(“123.msg”);
foreach (MapiAttachment att in msg.Attachments)
if (att.Properties.Contains(0x7FFD0003) && att.Properties[0x7FFD0003].GetInt32() == 8)
Console.WriteLine("Embedded, inline or linked Attachment File Name: " + att.LongFileName);
Console.WriteLine("Regular Attachment File Name: " + att.LongFileName);
The MapiMessage class is a specialized class, built to work with only Outlook Message format(.msg). If working with Outlook messages from any Outlook version, the results produced with MapiMessage class would be more concrete. On the contrast, MailMessage is generic enabling you work with all email message formats (.msg,.eml,.mhtml).
I would suggest you to use MapiMessage class if your input/output is of .MSG format.