Debugging MailHog in Docker: The Networking Fix I Wish I Knew Earlier

I ran into a MailHog Docker networking issue while testing a WordPress Core ticket, I needed to verify email functionality in my local development environment using MailHog. I am using the wordpress-develop Docker setup, so I assumed setting up MailHog would be simple.

To test it, I first installed the Check & Log Email plugin. I tried sending a test email, and it failed but without giving much detail. All I could see was an error as shown in the screenshot below.

That wasn’t enough to debug and that is what AI tool suggested. So I switched to WP Mail SMTP, which provided detailed SMTP error logs.

Versions:
WordPress: 6.9-alpha-60093-src
WordPress MS: No
PHP: 8.2.15
WP Mail SMTP: 4.5.0

Params:
Mailer: smtp
Constants: No
ErrorInfo: SMTP Error: Could not connect to SMTP host. Failed to connect to serverSMTP server error: Failed to connect to server Additional SMTP info: php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known
Host: mailhog
Port: 1025
SMTPSecure: string(0) ""
SMTPAutoTLS: bool(false)
SMTPAuth: bool(false)

Server:
OpenSSL: OpenSSL 1.1.1w 11 Sep 2023

Debug:
Email Source: WP Mail SMTP
Mailer: Other SMTP
SMTP Error: Could not connect to SMTP host. Failed to connect to serverSMTP server error: Failed to connect to server Additional SMTP info: php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known

SMTP Debug:
2025-06-30 17:37:41 Connection: opening to mailhog:1025, timeout=30, options=array()

2025-06-30 17:37:41 Connection failed. Error #2: stream_socket_client(): php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known [/var/www/src/wp-includes/PHPMailer/SMTP.php line 412]

2025-06-30 17:37:41 Connection failed. Error #2: stream_socket_client(): Unable to connect to mailhog:1025 (php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known) [/var/www/src/wp-includes/PHPMailer/SMTP.php line 412]

2025-06-30 17:37:41 SMTP ERROR: Failed to connect to server: php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known (0)

SMTP Error: Could not connect to SMTP host. Failed to connect to server

So here we have more information to debug the issue. But before we dive into the debugging I want to make sure to mention the real part: I wouldn’t have figured this out on my own. With the help of AI (yes, ChatGPT!), I walked through debugging, inspected Docker networks, tweaked docker-compose.override.yml, and eventually got everything working.

This post is for anyone who’s:

• Running WordPress in Docker
• Using MailHog to test email
• And getting nowhere with vague connection errors

Let’s walk through what went wrong and how I was able to fix it with AI tools.

The Setup: WP + MailHog + Docker + WP Mail SMTP

I had the following setup:

• WordPress running via Docker Compose
• MailHog container exposed on port 1025
• Email testing plugins:
  • Check & Log Email (used first, limited insight)
  • WP Mail SMTP (used next, provided full debug output)
• SMTP settings:
  • Host: mailhog
  • Port: 1025
  • Encryption: None
  • Auth: Off

The Error Part

The key error I got from WP Mail SMTP was:

SMTP Error: Could not connect to SMTP host. php_network_getaddresses: getaddrinfo for mailhog failed: Name or service not known

And once I also facd the below error:

STARTTLS command failed: 500 Unrecognised command

That means:

• MailHog was unreachable from the container
• Or MailHog didn’t support TLS

The Real Issue: Docker Network Mismatch

After a detailed debugging it was found out that:

• My PHP/WordPress container was on the wordpress-develop_wpdevnet network
• My MailHog container was on the default bridge network

So when WordPress tried to send mail using mailhog:1025, it failed because DNS resolution doesn’t work across isolated Docker networks.

The Fix: Make the Containers Talk

To fix this, it was mandatory to make sure both containers shared the same Docker network.

Step 1: Create a Shared Docker Network

docker network create wpdevnet

If this already exists (as part of wordpress-develop), you’ll see a message and can move on.

Step 2: Update docker-compose.override.yml

Add this file (or update if it exists) in your root project directory. But the safest option is to create the new one.

services:
  mailhog:
    image: mailhog/mailhog
    ports:
      - "8025:8025"   # MailHog Web UI
      - "1025:1025"   # SMTP
    networks:
      - wpdevnet

networks:
  wpdevnet:
    external: true
    name: wpdevnet

This tells Docker to run the mailhog container on the same custom network used by the main WordPress containers.

Step 3: Restart Everything

docker compose down --volumes

docker compose up -d

Step 4: Configure WP Mail SMTP and Save the changes

Inside WordPress:

• Mailer: Other SMTP
• SMTP Host: mailhog
• Port: 1025
• Encryption: None
• Auto TLS: Off
• Authentication: Off

Do NOT use TLS — MailHog doesn’t support encryption.
If you do, you’ll get this error:

STARTTLS command failed: 500 Unrecognised command

Success: Emails Now Appear in MailHog

Test emails from WordPress showed up in the MailHog UI at http://localhost:8025

WP Mail SMTP logs showed clean delivery. No more guessing; just working local email testing 🎉

What I Learned (and Why It Matters)

• Docker containers can only resolve hostnames if they’re on the same network ✅
• MailHog is a brilliant tool but lacks TLS, so you need to configure accordingly
• Sometimes the problem is not in code, but your environment setup
• And yes — AI can be your debugging buddy

Using ChatGPT helped me understand not just what to change, but why it mattered. It didn’t give me a copy-paste answer rather it gave me confidence to keep digging in.

Without AI guiding me through logs, Docker commands, and debug paths, I might have spent hours stuck. Instead, I walked away with a new Docker concept understanding, a fix that works, and a blog post I’m proud to share. ✨