Configure direct-delivery incoming email for self-hosted sites with Mail-Receiver

Documentation Self-Hosting
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Hello! I have a weird issue where I’ve set this up following the guide, and it works great! However, something has gone wrong with outbound email, which I thought wouldn’t be affected by any of this. Sidekiq gives the following error for every attempted email (all of them stuck in the Retries list) since I turned on mail-receiver:

Jobs::HandledExceptionWrapper: Wrapped OpenSSL::SSL::SSLError: SSL_read: unexpected eof while reading

My searching leads me to believe that this is related to TLS somehow. I had uncommented the TLS-related lines in the .yml file, but re-commenting them didn’t fix the issue either. I tried the instructions in the guide for resolving Postfix conflicts, but I apparently don’t have Postfix? (The /etc/postfix directory in the guide doesn’t exist on my instance, nor does it recognize postfix as a service.) And according to the netstat results, only docker-proxy is using port 25.

We’re using Gmail as the outbound SMTP service, and in fact we were using Gmail for inbound POP3 polling before this. I did delete a bunch of MX records pointing to Google, but the guide said to do that.

This is my mail-receiver.yml, with certain details redacted, of course:

## 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: discourse.[mydomain].org
# uncomment these (and the volume below!) to support TLS
  POSTCONF_smtpd_tls_key_file:  /letsencrypt/discourse.[mydomain].org/discourse.[mydomain].org.key
  POSTCONF_smtpd_tls_cert_file:  /letsencrypt/discourse.[mydomain].org/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://discourse.[mydomain].org'

  ## The master API key of your Discourse forum.  You can get this from
  ## the "API" tab of your admin panel.
  DISCOURSE_API_KEY: [myapikey]

  ## 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

Email tech is a bit out of my expertise, so I appreciate any advice, even if it’s to point out that I missed something stupid when I was setting this up. Thanks!