They originate from us. Looking at your account and our email server it shows both recent invites you sent were delivered to the recipients by their email server. Possibly they went to their spam or trash folder.
If they are still having issues getting them they can create free Plex accounts first then when you resend them (you will need to revoke the current ones first) they can just accept at https://app.plex.tv/desktop#!/settings/users-sharing