Example: If the attachment is itself a message (.pst and .ost formats only)

If the attachment is itself a message, you can read it using GetAttachmentMessage(attachment) of the BaseReader class. See the example below.

//A pst file containing message that contains attached message in Inbox folder.
string pstFileLocation = @"C:\test\someone@smthng.pst";
using (IReader reader = ReaderFactory.GetReader(pstFileLocation))
{

    Folder inboxFolder = reader.GetFolder(@"/Top of Outlook data file/Inbox");

    IEnumerable<Message> msgs = reader.ReadMessages(inboxFolder);
    foreach (Message msg in msgs)
    {
        //Display some of Message properties.
        Console.WriteLine($"{msg.EntryID} / {msg.Subject} / {msg.From} / {msg.DeliveryTime}");

        if (msg.HasAttachments)
        {
            foreach (Attachment attachment in msg.Attachments)
            {
                if (attachment.AttachMethod == AttachMethod.ATTACH_EMBEDDED_MSG)
                {
                    Message attMsg = (reader as BaseReader).GetAttachmentMessage(attachment);

                    //Write a msg file to the file system.
                    using (FileStream fs = new FileStream($"{attachmentSaveFolderPath}{attachment.SafeAttachmentFileName}", FileMode.Create, FileAccess.Write, FileShare.None)) {
                        reader.WriteAttachment(attachment, fs);
                    }
                }
            }
        }
    }
}