Delivering mail from host running the docker.

[henriec]

Hi, i did a lot of searching and reading and maybe i'm overlooking something, but i can't find how to get my host mail delivered to the docker. To clarify what i mean:

I have a server someserver.mydomain.com which runs a bunch of dockers. One of them is the DMS docker with the "virtual?" hostname myownmailserver.mydomain.com . Every is working fine, certificates DKIM, SPF, mail delivery mail receiving etc etc.

On the server hosting the docker there are some cron tasks which eventually can/may produce some output. How can i get the output of the host-cron-jobs to the docker. Installing an MTA on the host feels like "defeats -the-purpose", besides that, the ports for a full MTA are in use by the DMS docker.

[henriec]

I came up with this simple solution... Anyone conmments?

It works like a charm...

At the host:
# ls -larth /usr/sbin/sendmail
-rwxr-xr-x 1 root root 120 Mar 27 17:17 /usr/sbin/sendmail

# cat  /usr/sbin/sendmail
#!/bin/bash

cat <&0 | docker compose -f /opt/docker/mailserver/compose.yaml exec -T mailserver /usr/sbin/sendmail -t 

[casperklein]

Alternatively, take a look at "nullmailer". It also provides a sendmail binary on the host.

[polarathene]

Installing an MTA on the host feels like "defeats -the-purpose", besides that, the ports for a full MTA are in use by the DMS docker.

You just need a mail client, which as you noticed can be run from within a container, so a wrapper script works.

It's a bit more tricky when trying to deliver mail via port 25 as that has security checks in place, but you can permit this internally of the DMS container via PERMIT_DOCKER=container, and then using swaks.

Alternatively you can create an account in DMS specifically for sending such mail and use those credentials from the host (or any other system) to connect over port 587 or 465, which bypasses security checks for port 25 (which should typically only be MTA traffic). If you go this route you may want the ENV SPOOF_PROTECTION=1 enabled to forbid this account from being able to send with any sender address.

The sendmail solution you chose AFAIK avoids ports and will use the mail drop location in Postfix to queue mail for sending without having to deliver into Postfix via SMTP.