In case it helps, here is my working mail-receiver.yml file contents.
I use Cloudflare SSL, so since Cloudflare is providing the TLS I left all of the TLS lines commented.
## this is the incoming mail receiver container template
##
## After making changes to this file, you MUST rebuild
## /var/discourse/launcher rebuild mail-receiver
##
## BE *VERY* CAREFUL WHEN EDITING!
## YAML FILES ARE SUPER SUPER SENSITIVE TO MISTAKES IN WHITESPACE OR ALIGNMENT!
## visit http://www.yamllint.com/ to validate this file as needed
base_image: discourse/mail-receiver:release
update_pups: false
expose:
- "25:25" # SMTP
env:
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LANGUAGE: en_US.UTF-8
## Where e-mail to your forum should be sent. In general, it's perfectly fine
## to use the same domain as the forum itself here.
MAIL_DOMAIN: mydiscourseinstance.com
# uncomment these (and the volume below!) to support TLS
# POSTCONF_smtpd_tls_key_file: /letsencrypt/discourse.example.com/discourse.example.com.key
# POSTCONF_smtpd_tls_cert_file: /letsencrypt/discourse.example.com/fullchain.cer
# POSTCONF_smtpd_tls_security_level: may
## The base URL for this Discourse instance.
## This will be whatever your Discourse site URL is. For example,
## https://discourse.example.com. If you're running a subfolder setup,
## be sure to account for that (ie https://example.com/forum).
DISCOURSE_BASE_URL: 'https://mydiscourseinstance.com'
## The master API key of your Discourse forum. You can get this from
## the "API" tab of your admin panel.
DISCOURSE_API_KEY: xxxxxxxxxxxxxxxx
## The username to use for processing incoming e-mail. Unless you have
## renamed the `system` user, you should leave this as-is.
DISCOURSE_API_USERNAME: system
volumes:
- volume:
host: /var/discourse/shared/mail-receiver/postfix-spool
guest: /var/spool/postfix
# uncomment to support TLS
# - volume:
# host: /var/discourse/shared/standalone/letsencrypt
# guest: /letsencrypt