We are trying to create an MSG using a custom Recipient Address Type (Recipient Table -> PR_ADDRTYPE) for a faxing integrated solution. The problem is that MapiMessage.AddressType is readonly, so we assign it using the MapiProperty, but it doesn't get carried over into the generated MSG when opened via Outlook - it still shows SMTP as the address type.
MapiProperty propAddressType = new MapiProperty(MapiPropertyTag.PR_ADDRTYPE, Encoding.UTF8.GetBytes("MYFAX"));
recipientTo.SetProperty(propAddressType);
This MAPI field corresponds to the Recipients Table PR_ADDRTYPE field. In Outlook 2007-2013, this is managed via right-clicking on the email address and selecting "Open Outlook Properties". The default Email Type is SMTP, but it can be modified for custom message routing - in our case faxing.
I've attached the expected result after clicking the "Custom Type" button and editing the email type.