I really appreciate you for going above and beyond as a GOM--despite this being an unpaid job. Having to resolve this unfortunate issue is difficult, since you did your best and there's really no one at fault here. The fact that you reassured your joiners and even exerted this much effort speaks a lot for your reputation as a GOM and as a person. Like anon said, "don't beat youself" too much. Undelivered or lost mails are risks we take as joiners. I still do hope they safely reach everyone's homes though.