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A bring-your-own-SMTP-server mail relay

πŸ“„ Description

As a developer, chances are high that at some point you need to teach your application how to send mails. Essentially, there are two options. Either you use a professional mail sending service or you include an SMTP client library to your software and plug your own mail server.

Think of MailWhale like Mailgun, SendGrid or SMTPeter, but open source and self-hosted. Or like Postal or Cuttlefish, but less bloated and without running its own, internal SMTP server.

However, if you want the best of both worlds – that is, send mails via simple HTTP calls and with no extra complexity, but still use your own infrastructure – you may want to go with βœ‰οΈπŸ³.

You get a simple REST API, which you can call to send out e-mail. You can plug your self-hosted SMTP server, as well as Google Mail or literally any other e-mail provider.

Stay tuned, there is a lot more to come.

🚧 Project State

The project is in a very early stage and breaking changes are likely to happen. We'd recommend to not yet use this in production or at least expect non-trivial effort required to upgrade to a new version.

For a more stable and robust alternative to MailWhale, check out postalsys/emailengine.

πŸ“¦ Installation

Compile from source

# 1. Clone repo
$ git clone

# 2. Adapt config to your needs, i.e. set your SMTP server and credentials, etc.
$ cp config.default.yml config.yml
$ vi config.yml

# 3. Compile Web UI
$ cd webui/
$ yarn && yarn build
$ cd ..
# 4. Compile API
$ go build

# 5. Run it
$ ./mailwhale

From GitHub Release

# 1. Download latest release
curl -s | jq -r ".assets[] | select(.name|match(\"Linux_$(arch).tar.gz\")) | .browser_download_url" | wget -qi -

# 2. Extract
mkdir mailwhale
tar xf mailwhale_*.tar.gz -C mailwhale
cd mailwhale

# 3.[Optional] Adapt config to your needs, i.e. set your SMTP server and credentials, etc.
# vi config.yml

# 4. Run it

With Docker Image

$ docker run -d \
  -p \
  -v "$(pwd)/config.yml":/app/config.yml:ro \
  -v mailwhale_data:/data \
  --name mailwhale \
  docker pull

Build custom Docker Image

# 1. Clone repo
$ git clone

# 2. Adapt config to your needs, i.e. set your SMTP server and credentials, etc.
$ cp config.default.yml config.yml
$ vi config.yml

# 3. Build image
$ docker build -t mailwhale .

# 4. Create persistent volume
$ docker volume create mailwhale_data

# 5. Run
$ docker run -d \
  -p \
  -v "$(pwd)/config.yml":/app/config.yml:ro \
  -v mailwhale_data:/data \
  --name mailwhale \

Note: An official Docker image is about to come. Also, there will be no need to mount your config file into the container, as everything will be configurable using environment variables eventually.

Reverse Proxy

To run this app behind a reverse proxy, see here for example configurations for different web servers (works analogously for MailWhale). In addition, MW_WEB_PUBLIC_URL (or web.public_url, respectively) must be configured accordingly (set to the absolute URL of your MailWhale instance). Also see #43.

⌨️ Usage

First of all, you can get most tasks done through the web UI, available at http://localhost:3000.

1. Define a user

To get started with MailWhale, you need to create a user first. To do so, register a new user API or web UI. security.allow_signup needs to be set to true.

2. Create an API client

It is good practice to not authenticate against the API as a user directly. Instead, create an API client with limited privileges, that could easily be revoked in the future. A client is identified by a client ID and a client secret (or token), very similar to what you might already be familiar with from AWS APIs. Usually, such a client corresponds to an individual client application of yours, which wants to access MailWhale's API.


$ curl -XPOST \
     -u '' \
     -H 'Content-Type: application/json' \
     --data-raw '{
         "description": "My juicy web app",
         "sender": "Epic Juice Store <>",
         "permissions": ["send_mail"]
     }' \


    "id": "SVNORFBUWGhxWGZSUUl0eA==",
    "description": "My juicy web app",
    "permissions": [
    "sender": "Epic Juice Store <>",
    "api_key": "75c74447-c4af-453b-ad06-3a8ae969ed16"

The response contains your new client's ID (id) and secret (api_key). Remember these credentials, as they are needed for subsequent requests from your application.

Client authentication happens through HTTP basic auth. Most HTTP clients support basic auth out of the box (including cURL with its -u parameter). If your's doesn't, you can hash create the hash like so:

$ echo "Authorization: Basic $(echo '<client_id>:<client_secret>' | base64)"

# Result:
# Authorization: Basic U1ZOT1JGQlVXR2h4V0daU1VVbDBlQT09Ojc1Yzc0NDQ3LWM0YWYtNDUzYi1hZDA2LTNhOGFlOTY5ZWQxNg==

3. Send E-Mails

Plain text or HTML

$ curl -XPOST \
  -u '<client_id>:<client_secret>' \
  -H 'content-type: application/json' \
  --data '{
      "to": ["Jane Doe <>"],
      "subject": "Dinner tonight?",
      "html": "<h1>Hey you!</h1><p>Wanna have dinner tonight?</p>"
  }' \

You can also a text field instead, to send a plain text message.

Using a template

In case you have created a template using the web UI, you can reference it in a new mail like so:

$ curl -XPOST \
  -u '<client_id>:<client_secret>' \
  -H 'content-type: application/json' \
  --data '{
      "to": ["Jane Doe <>"],
      "subject": "Dinner tonight?",
      "template_id": "8033ea08-2630-408b-82f9-d38b403243d0",
      "template_vars: {
        "text.greeting": "Hello new user!",
  }' \

πŸ”§ Configuration Options

You can specify configuration options either via a config file (config.yml) or via environment variables. Here is an overview of all options.

YAML Key Environment Variable Default Description
env MW_ENV dev Whether to use development- or production settings
mail.domain MW_MAIL_DOMAIN - Default domain for sending mails
web.listen_addr MW_WEB_LISTEN_ADDR IP and port for the web server to listen on (can be IPv4 or IPv6)
web.cors_origin - [http://localhost:5000] List of URLs which to accept CORS requests for
web.public_url MW_PUBLIC_URL http://localhost:3000 The URL under which your MailWhale server is available from the public internet MW_SMTP_HOST - SMTP relay host name or IP
smtp.port MW_SMTP_PORT - SMTP relay port
smtp.username MW_SMTP_USER - SMTP relay authentication user name
smtp.password MW_SMTP_PASS - SMTP relay authentication password
smtp.tls MW_SMTP_TLS false Whether to require full TLS (not to be confused with STARTTLS) for the SMTP relay
smtp.skip_verify_tls MW_SMTP_SKIP_VERIFY_TLS false Whether to skip certificate verification (e.g. trust self-signed certs)
store.path MW_STORE_PATH ./data.json.db Target location of the database file
security.pepper MW_SECURITY_PEPPER - Pepper to use for hashing user passwords
security.allow_signup MW_SECURITY_ALLOW_SIGNUP true Whether to allow the registration of new users
security.verify_users MW_SECURITY_VERIFY_USERS true Whether to require new users to activate their account using a confirmation mail
security.verify_senders MW_SECURITY_VERIFY_SENDERS true Whether to validate sender addresses and their domains' SPF records
security.block_list MW_SECURITY_BLOCK_LIST [] List of regexes used to block certain recipient addresses

Sender verification & SPF Check

By default, mails are sent using a randomly generated address in the From header, which belongs to the domain configured via mail.domain (i.e. Optionally, custom sender addresses can be configured on a per-API-client basis. However, it is recommended to properly configure SPF on that custom domain and instruct MailWhale to verify that configuration.

As a user, you need to configure your domain, which you want to use as part of your senders address (e.g. for sending mails from User Server <>), to publish an SPF record that delegates to the domain under which MailWhale is running (e.g.  IN  TXT v=spf1

As a server operator of a MailWhale instance, you need to enable mail.verify_senders and set your mail.domain and web.public_url. For that domain, you need to configure an SPF record that allows your SMTP relay provider's (e.g., GMail, SendGrid, etc.) mail servers to be senders. Refer to your provider's documentation, e.g. this.

πŸš€ Features (planned)

Right now, this app is very basic. However, there are several cool features on our roadmap.

  • Bounce handling: Ultimately, we want to offer the ability to plug an IMAP server in addition, to get notified about bounced / undelivered mails.
  • Statistics: There will be basic statistics about when which client has sent how many mails, how many were successful or were bounced, etc.
  • Client libraries: To make the developer experience even smoother, client SDKs for different programming languages will we added some time.
  • Minor enhancements: IPv6- and TLS support, API documentation, ...

πŸ““ License