I set these message properties:
PR_SUBJECT;
PR_BODY;
PR_SENDER_EMAIL_ADDRESS;
PR_SENDER_NAME;
PR_SENDER_ADDRTYPE;
PR_SENT_REPRESENTING_EMAIL_ADDRESS;
PR_SENT_REPRESENTING_NAME;
PR_SENT_REPRESENTING_ADDRTYPE;
PR_MESSAGE_FLAGS;
PR_CLIENT_SUBMIT_TIME;
PR_CREATION_TIME;
PR_MESSAGE_CLASS;
PR_EMAIL_ADDRESS;
PR_DISPLAY_TO;
PR_DEFAULT_PROFILE;
PR_AUTO_FORWARDED;

and for recipient:
PR_RECIPIENT_TYPE
PR_ADDRTYPE
PR_EMAIL_ADDRESS
PR_DISPLAY_NAME

// Message creation:
hr = pMAPILogonEx( 0L, NULL, NULL, ulFlags, &spIMAPISession );
hr = spIMAPISession-OpenMsgStore( ..., MAPI_BEST_ACCESS | MDB_WRITE,
&spIMsgStore );
hr = spIMsgStore-OpenEntry( ..., MAPI_BEST_ACCESS, ..., &spSentFolder );
hr = spSentFolder-CreateMessage( NULL, 0, &spMsg );
// ... filling properrties here
hr = spMsg-SaveChanges( FORCE_SAVE );