Domain Name System – Postfix in Docker: Host or Domain Name Not Found

I'm running Postfix 3.4.8 on Debian 10 (buster) in Docker 19.03.5. Postfix is configured as a smarthost. It relays emails to the SMTP server of our ISP.

If I configure the relayhost setting with the domain name of the SMTP server, in my case [mail.level27.be]:587, then I get this error when sending mail:

Host or domain name not found. Name service error for name=mail.level27.be type=A: Host not found, try again

If I configure the relayhost setting with the IP of the SMTP server, in my case [185.3.216.85]:587, it works!

I guess it's a DNS issue. How can I make it work with the domain name in stead of the IP?

The Docker image is build with this command:

docker build --pull -t mypostfix .

Here is the Dockerfile:

FROM debian:10
EXPOSE 25

RUN apt-get update
RUN apt-get -y install libsasl2-modules postfix

RUN postconf -X 'mynetworks' \
    && postconf -e 'mynetworks_style = subnet' \
    && postconf -X 'mydestination' \
    && postconf -e 'remote_header_rewrite_domain =' \
    && postconf -e 'inet_protocols = ipv4' \
    && postconf -e 'relayhost = [mail.level27.be]:587' \
    && postconf -e 'smtp_use_tls = yes' \
    && postconf -e 'smtp_sasl_auth_enable = yes' \
    && postconf -e 'smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd' \
    && postconf -e 'smtp_sasl_security_options = noanonymous' \
    && postconf -e 'maillog_file = /dev/stdout' \
    && echo "mail.level27.be   username:password" > /etc/postfix/sasl_passwd \
    && chown root:root /etc/postfix/sasl_passwd \
    && chmod 600 /etc/postfix/sasl_passwd \
    && postmap /etc/postfix/sasl_passwd

CMD postfix start-fg

The Docker container is started with this command:

docker run --rm -d -p 25:25 mypostfix

The email is sent on the Docker host with this curl command:

echo "This is some mail content." > /tmp/email.txt

curl --url 'smtp://localhost:25' --mail-from 'xxx.yyy@zzz.be' --mail-rcpt 'xxx.yyy@zzz.be' --upload-file /tmp/email.txt

The error can be seen by running postqueue -p in the container:

root@mypostfix:/# postqueue -p
-Queue ID-  --Size-- ----Arrival Time---- -Sender/Recipient-------
DBB26A05886      256 Wed Feb 19 14:45:42  xxx.yyy@zzz.be
(Host or domain name not found. Name service error for name=mail.level27.be type=A: Host not found, try again)
                                         xxx.yyy@zzz.be
-- 0 Kbytes in 1 Request.