Sadly I cannot help. This is a very similar situation to one I previously attempted to fix via Gmail, but there’s no easy fix. Mail from Discourse has a “mailing list” header, as the email is auto-generated. As one of our engineers explained to me when I asked why:
Because they’re not human-generated. Good mail handling hygiene dictates that you not send autoreplies to machine-generated e-mails, because that is how mail loops form (machine A sends a notification to machine B, machine B sends autoreply to machine A, machine A sends autoreply to machine B, and so on).
To resolve this you’d need to either A) create a Discourse Plugin that can handle this or B) create your own autoresponder service and hook it up to this user. If you choose to do either, be careful to avoid creating a mail loop.