Adding the docker network's gateway to the list of trusted hosts, e.g. using the `network` or `connected-networks` option, can create an [**open relay**](https://en.wikipedia.org/wiki/Open_mail_relay), for instance [if IPv6 is enabled on the host machine but not in Docker][github-issue-1405-comment].
Make sure to set the proper `domainname` that you will use for the emails. We forward only SMTP ports (not POP3 and IMAP) because we are not interested in accessing the mailserver directly (from a client). We also use these settings:
-`PERMIT_DOCKER=network` because we want to send emails from other docker containers.
-`SSL_TYPE=letsencrypt` because we will manage SSL certificates with letsencrypt.
- We need to open ports `25`, `587` and `465` on the firewall:
- Now generate the DKIM keys with `./setup.sh config dkim` and copy the content of the file `config/opendkim/keys/domain.tld/mail.txt` on the domain zone configuration at the DNS server. I use [bind9](https://github.com/docker-scripts/bind9) for managing my domains, so I just paste it on `example.org.db`:
```txt
mail._domainkey IN TXT ( "v=DKIM1; h=sha256; k=rsa; "
"iqq3bD/BVlwKRp5gH6TEYEmx8EBJUuDxrJhkWRUk2VDl1fqhVBy8A9O7Ah+85nMrlOHIFsTaYo9o6+cDJ6t1i6G1gu+bZD0d3/3bqGLPBQV9LyEL1Rona5V7TJBGg099NQkTz1IwIDAQAB" ) ; ----- DKIM key mail for example.org
- Get an SSL certificate from letsencrypt. I use [wsproxy](https://gitlab.com/docker-scripts/wsproxy) for managing SSL letsencrypt certificates of my domains:
./setup.sh alias add admin@example.org myemail@gmail.com
./setup.sh alias add info@example.org myemail@gmail.com
./setup.sh email list
./setup.sh alias list
```
Aliases make sure that any email that comes to these accounts is forwarded to my real email address, so that I don't need to use POP3/IMAP in order to get these messages. Also no anti-spam and anti-virus software is needed, making the mailserver lighter.
This extra step is required to avoid the `553 5.7.1 Sender address rejected: not owned by user` error (the account used for setting up Gmail is `admin.gmail@example.org` and `info.gmail@example.org` )
- Send some test emails to these addresses and make other tests. Then stop the container with `ctrl+c` and start it again as a daemon: `docker-compose up -d mail`.
- Now save on Moodle configuration the SMTP settings and test by trying to send some messages to other users: