Thanks so much for putting together the Direct-reply-to email. I think it’s one of the most underrated features of Discourse!
The guide is excellent. However, one minor yet error-prone detail is letsencrypt key file setting. It should be very obvious that it’s a placeholder, eg <your domain key file>.
# uncomment these (and the volume below!) to support TLS
# POSTCONF_smtpd_tls_key_file: /letsencrypt/discourse.example.com/prop.ltcmp.net.key <---- ???
# POSTCONF_smtpd_tls_cert_file: /letsencrypt/discourse.example.com/fullchain.cer
# POSTCONF_smtpd_tls_security_level: may