issue with mailing list delivery when using external SMTP server

[avatar1024]

Hello all,

I've recently set-up an external SMTP relay using forwardemail. It works well, emails are going through etc.

However I have encountered a problem using Cloudron mailing lists:

When sending an email from en external email address (e.g. @posteo.net) to a Cloudron list which contains addresses @mydomain.coop (the domain on Cloudron) and some other adresses, e.g. @gmail.com or @posteo.net I am getting bounces for all recipients with those external addresses. Delivery works fine for the recipients @mydomain.coop and all works fine to ALL recipients (no bounces) if the sender is an address @mydomain.coop.

Bounces look like:

Hi. This is the Mailer program at mydomain.coop.

I'm afraid I wasn't able to deliver your message
    "test email"
to the following addresses.

This is a permanent error; I've given up. Sorry it didn't work out.

Intended Recipients: <some.person@gmail.com>
Failure Reason: Error: 550 From header must end with @mydomain.coop

[avatar1024]

Addition email server logs:

When sending from @mydomain.coop with successful delivery:

{
  "ts": 1751965332978,
  "type": "delivered",
  "direction": "outbound",
  "uuid": "425FB09B-28B2-4DFD-B19D-24763E77D8A5.1.1",
  "messageId": "<bc8821b5-e1c3-422c-bce0-3be7ae6b974b@mydomain.coop>",
  "mailFrom": "<someone@mydomain.coop>",
  "spamStatus": null,
  "mailbox": null,
  "quotaPercent": null,
  "rcptTo": [
    "<some.person@gmail.com>"
  ],
  "server": {
    "host": "149.28.215.223",
    "ip": "149.28.215.223",
    "port": 587
  },
  "response": "OK: message queued"
}

When sending from an external address to the same list and failing on the same recipient:

{
  "ts": 1751964710037,
  "type": "bounce",
  "direction": "outbound",
  "uuid": "4C06313B-F1B3-460A-AC38-59402F3C3ACE.1.1",
  "messageId": "<76622dc4-1d22-4fe3-a863-e5da6a39a0b8@posteo.net>",
  "mailFrom": "<someone@posteo.net>",
  "spamStatus": null,
  "mailbox": null,
  "quotaPercent": null,
  "rcptTo": [
    "<some.person@gmail.com>"
  ],
  "message": "550 From header must end with @mydomain.coop",
  "mx": {
    "exchange": "149.28.215.223",
    "priority": 0,
    "port": 587,
    "auth_user": "*@mydomain.coop",
    "auth_pass": "c3b478557ae86e1b599aba64",
    "auth_type": "plain",
    "bind_helo": "my.domain.coop",
    "from_dns": "smtp.forwardemail.net"
  }
}

[james]

Hello @avatar1024 could you share your real Cloudron domain so I can take a look into the DNS records for MX, DMARC and so on?

[avatar1024]

@james sure it's catalystcollective.org

For clarity, sending/receiving emails works fine, it's only when someone with an external email address (i.e. not from this domain) sends an email through a cloudron list that delivery fails for recipients (list members) who also have external addresses.

[girish]

I think this has been reported previously about mailing list. Cloudron uses SRS but gmail and friends don't support SRS. I don't have an answer but we have to investigate on our side whether we can implement mailing list "reliably". So far, it's a low priority since the main use case for mailing list is internal mail distribution and it's not a real mailing list like the kernel mailing lists with external people . The feature initially came from Groups feature of Cloudron which was just forwarding mail to internal users.

[avatar1024]

@girish Hi Girish, I see the issue you mean (I also reported it a while ago, it was about double forward) but this one is different (although the cause might be same). Before switching to the external SMTP the mailing list was working fine for this use case, it's only after switching that it started happening.

While the other issue was a bit niche, this issue / use case seems more critical as it affects the basic functioning of mailing lists when using an external smtp.

[sponch]

Same here (since sending through Brevo (Sendinblue)).