no MX host for [domain] has a valid address record

[Vlad GURDIGA]

Hello! 👋🙂

I’m frequently seeing warning like this in my Postfix logs:

2023-05-08T06:02:56.631985+00:00 WARNING postfix/smtp[15075]: warning: no MX host for frogjump.co.uk has a valid address record

It happens every couple of days, and so I’ve made myself a mechanism to debug this issue, and here is what I found: 🤓

My Postfix runs inside a Docker container, so I have a script that watches the log (tail -f FWT!), and when I see this error, I immediately run a few nslookup queries, in this order:

0. "A" record for google.com from inside the container — a general sanity check;
1. "A" record from inside the container;
2. "MX" record from inside the container;
3. the above three queries from the Docker host.

I have beed repeatedly surprised in the last couple of weeks when all of the above queries would resolve successfully (output in the PS below). 🤔

QUESTION: I suppose this is not related to Postfix, but is some sort of temporary DNS glitch at the OS level, and so I’m wondering what would be a proper approach to gracefully handle it?

MORE INFO: My system delivers around 1.5 emails a day, and I only see one of these warnings every other day or so.

PS: Just for reference, below is the output from my lil script:

--- DEBUG: google.com from inside container
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
Name:   google.com
Address: 172.217.18.14

Non-authoritative answer:
Name:   google.com
Address: 2a00:1450:4001:80b::200e

--- DEBUG: A record from inside container
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:

Non-authoritative answer:
Name:   frogjump.co.uk
Address: 69.5.8.25

--- DEBUG: MX record from inside container
Server:         127.0.0.11
Address:        127.0.0.11:53

Non-authoritative answer:
frogjump.co.uk  mail exchanger = 1 mx.futurequest.net

--- DEBUG: google.com from host
Server:         1.1.1.1
Address:        1.1.1.1#53

Non-authoritative answer:
Name:   google.com
Address: 142.250.181.238
Name:   google.com
Address: 2a00:1450:4001:80e::200e

--- DEBUG: A record from host
Server:         1.1.1.1
Address:        1.1.1.1#53

Non-authoritative answer:
Name:   frogjump.co.uk
Address: 69.5.8.25

--- DEBUG: MX record from host
Server:         1.1.1.1
Address:        1.1.1.1#53

Non-authoritative answer:
frogjump.co.uk  mail exchanger = 1 mx.futurequest.net.

Authoritative answers can be found from: