📨 How to Set Up Imapsync in Docker with Caddy v2

(and Why This Method Works)

Imapsync is a powerful tool for syncing mailboxes from one IMAP server to another. This guide will show you how to set it up in a Docker container and securely serve its web interface using Caddy v2 as a reverse proxy with HTTPS.

✅ What You’ll Achieve

• Run gilleslamiral/imapsync in Docker
• Securely access Imapsync via a subdomain (e.g., https://imapsync.yourdomain.com)
• Automatically provision an SSL certificate with Let’s Encrypt using Caddy v2
• Use Docker internal networking to keep things clean and secure

🛠️ Prerequisites

• A Docker host with Portainer (or Docker CLI)
• A domain name (e.g., yourdomain.com)
• DNS pointing imapsync.yourdomain.com to your Docker host’s public IP
• Caddy v2 running as a Docker container with its config volume mounted

🧱 Step 1: Create a Docker Network

We’ll use a shared Docker network (caddy-net) to allow containers to talk to each other by service name.

docker network create caddy-net

🐳 Step 2: Deploy Imapsync in Docker

Use this configuration in Portainer (or a docker run command):

✅ Key Points

• The container listens on ports 8080 (HTTP) and 8443 (HTTPS)
• You don’t need to expose those ports to the host if you’re using Caddy
• The container name (or alias in the network) will be imapsync

Example CLI:

docker run -d \
  --name imapsync \
  --network caddy-net \
  gilleslamiral/imapsync

In Portainer:

• Set the container name: imapsync
• Network: attach to caddy-net
• You do not need to publish ports (unless you want external access for testing)

🌐 Step 3: Update Caddyfile

Here’s the Caddy v2 configuration block:

imapsync.yourdomain.com {
    reverse_proxy https://imapsync:8443 {
        transport http {
            tls_insecure_skip_verify
        }
    }
    encode gzip
    tls you@example.com
}

✅ Why This Works

• https://imapsync:8443: We proxy over HTTPS to the container (internal Docker DNS resolves imapsync)
• tls_insecure_skip_verify: The container uses a self-signed cert (for docker.lamiral.info), so we skip verification
• encode gzip: Enables compression
• tls you@example.com: Required for automatic SSL provisioning with Let’s Encrypt

🔄 Step 4: Reload Caddy

If running via CLI:

docker exec -it caddy caddy reload --config /etc/caddy/Caddyfile

Or just restart the Caddy container:

docker restart caddy

🧪 Step 5: Test It

Visit your site:

https://imapsync.yourdomain.com

You should see the Imapsync web interface, secured with HTTPS via a valid Let’s Encrypt cert.

🔍 Common Issues & Fixes

🎓 Why Use This Method?

• Security: Imapsync exposes an HTTPS interface internally; we tunnel it through Caddy with a trusted cert.
• Simplicity: Caddy handles HTTPS, reverse proxy, and cert renewal automatically.
• Isolation: Only Caddy talks to the internet. Everything else stays inside Docker.

If you’d like this written up as a Markdown file, or ready to publish on a site like a knowledge base or blog, let me know!