There are some domains that will resolve on the unbound container that will not resolve on the postfix container. An example is mailcow.email.

I tried to sign up for this forum and I never got the email. I looked in the postfix logs and saw this:

6/29/2020, 11:55:28 AM	info	disconnect from hosted.mailcow.de[5.1.76.202] ehlo=2 starttls=1 mail=1 rcpt=0/1 data=0/1 rset=1 quit=1 commands=6/8
6/29/2020, 11:55:28 AM	info	NOQUEUE: reject: RCPT from hosted.mailcow.de[5.1.76.202]: 450 4.1.8 <community@mailcow.email>: Sender address rejected: Domain not found; from=<community@mailcow.email> to=<my_email@my_domain.com> proto=ESMTP helo=<hosted.mailcow.de>
6/29/2020, 11:54:57 AM	info	Anonymous TLS connection established from hosted.mailcow.de[5.1.76.202]: TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits)
6/29/2020, 11:54:57 AM	info	connect from hosted.mailcow.de[5.1.76.202]

What the heck do you mean “Domain not found”? I was just on the web site!

Does mailcow.email resolve on the postfix container?

root@my_server:/opt/mailcow-dockerized# docker-compose exec postfix-mailcow /bin/bash
root@my_server:/# nslookup mailcow.email
Server:         127.0.0.11
Address:        127.0.0.11#53

** server can't find mailcow.email: SERVFAIL

root@my_server:/#

Nope. Can other domains be resolved on the postfix container?

root@my_server:/# nslookup google.com
Server:         127.0.0.11
Address:        127.0.0.11#53

Non-authoritative answer:
Name:   google.com
Address: 172.217.3.174
Name:   google.com
Address: 2607:f8b0:400a:808::200e

root@my_server:/#

Yup. OK, will mailcow.email resolve on the unbound container?

root@my_server:/opt/mailcow-dockerized# docker-compose exec unbound-mailcow /bin/bash
bash-5.0# nslookup mailcow.email
Server:         127.0.0.11
Address:        127.0.0.11#53

Non-authoritative answer:
Name:   mailcow.email
Address: 5.1.76.57
Name:   mailcow.email
Address: 2a00:f820:417::e12:892d

bash-5.0#

Works just fine. So why doesn’t it resolve on the postfix container?