Thank you for clarification.
Aspose.Email does allow setting a custom header while sending an appointment, and these are visible in outlook if the appointment’s properties are checked in Outlook. However, it seems that you want to retrieve the sent appointment (through Aspose) using Outlook Interop. and facing some issue. Please confirm if I am right here?
Using the following code via Aspose API, I found this working when the sent appointment message’s properties are viewed in outlook.
// create the meeting request<o:p></o:p>
Appointment app = new
DateTime.Now.AddMinutes(30), "email@example.com", "firstname.lastname@example.org");
app.Summary = "meeting
app.Description = "description";
MailMessage msg = new
msg.From = "email@example.com";
msg.To = "firstname.lastname@example.org";
msg.IsBodyHtml = true;
msg.HtmlBody = "
Email Message detail";
msg.Subject = "meeting
// send the appointment
SmtpClient client = GetSmtpClient();