You should really let discourse handle all of it’s emails.
for sending emails, I’d assume you’ve configured an external SMTP (e.g. mailgun) and for receiving a standalone MTA (e.g. mail-receiver container). I don’t think discourse ever auto-replies to any email unless there are issues (e.g. incorrectly formatted email address).
If You follow the guides correctly, The setup looks like this:
All system emails are sent via noreply@discourse.example.com while the emails that expect a reply (notifications) are sent from reply+{reply_key}@discourse.example.com All the outbound mail is processed via your ESP and incoming is Processed via the MTA neither should have the ability to do the vice-versa (i.e. ESP can’t receive and MTA can’t send)