I am also running into this issue. Looks like there was no resolution. According to the RFC we shouldnt need the threadId as it is an internal Gmail concept. But Gmail does not seem to reliably thread emails together based solely on the headers above and the subject. Is Gmail not compliant with the RFC in this case? Using the threadId does not work because user1 doesnt have access to user2's thread. Very confusing. Hoping someone has solved this.