@ceene: Try adding participant to receipt, or send it directly to participant. I can't test it right now but maybe it works
OUTGOING: <receipt> id=w5rE/i4sYEkG1 to=39328xxxxxxx-1449917322@g.us type=error <error> type=plaintext-only OUTGOING: <receipt> id=w5rE/i4sYEkG1 to=39320xxxxxxx@s.whatsapp.net type=error <error> type=plaintext-only