Lists of required and optional properties are documented in several places:
IABContainer : IMAPIContainer
IAttach : IMAPIProp
IDistList : IMAPIContainer
IMailUser : IMAPIProp
IMAPIContainer : IMAPIProp
IMAPIStatus : IMAPIProp
IMessage : IMAPIProp
IMsgStore : IMAPIProp
IProfSect : IMAPIProp
Required Properties for All Messages
Required Report Message Properties
Optional Report Message Properties
Recipient Properties for All Messages
Recipient Properties for Delivery Status Reports