I'm using the below method to reply to mails coming in to a business function mailbox.
The body text being added is only intermittently being set. This method is only called when someone has emailed in to unsubscribe from a mailing but the email address of the sender (or in the body) hasn't been found in the database and we want to ask them to send us the mail address they want to unsubscribe.
private void replyToMail(OutlookItem item)
{
RDOSession session = new RDOSession();
session.Logon(null, null, null, true, null, null);
RDOMail thisItem = session.GetMessageFromID(item.EntryID, item.StoreID, null);
RDOMail reply = thisItem.Reply();
RDOAddressEntry optingout = session.AddressBook.GAL.ResolveName("optingout");
//reply.Sender = optingout; this had no effect
reply.SentOnBehalfOf = optingout;
reply.Subject = "Automated Response - Could not complete unsubscribe";
reply.Body = "This is an automated response from the Newsletter unsubscribe system. We couldn't find "+item.Sender+" in our database to unsubscribe you from our mailings.\r\n\r\nPlease reply to this mail and include the email address you want to unsubscribe.\r\n\r\nKind Regards\r\n.";
reply.Send();
session.Logoff();
}