Go SMTP Slacker

A simple, lightweight SMTP server that receives emails and forwards them as direct messages to Slack users. It's designed to be a simple bridge for applications that can only send notifications via email, allowing them to post messages to Slack instead.

Features

• Acts as a standard SMTP server.
• Parses incoming emails for sender, recipients, subject, and body.
• Forwards formatted messages as direct messages to Slack users, looking them up by the recipient email address.
• Supports optional SMTP PLAIN authentication.
• Filters sender and recipient addresses using flexible allow/deny lists.

Configuration

The server is configured using a YAML file (e.g., config.yaml). The file has two main sections: smtp, and slack.

Example config.yaml

smtp:
  listen-addr: "0.0.0.0:1025"
  auth:
    enabled: true
    user-database: "/etc/go-smtp-slacker/users.db"
  policies:
    from:
      default-action: "deny" # "allow" or "deny"
      allow:
        - "alerts@example.com"
        - "monitoring-?@my-domain.com"
      deny:
        - "*@spam.com"
    to:
      default-action: "allow"
      allow:
        - "*"
      deny:
        - "ignored@example.com"
slack:
  token: "xoxb-your-slack-bot-token"

---

smtp Section

This section configures the core SMTP server.

• addr: The IP address and port the server should listen on. Use 0.0.0.0 to listen on all network interfaces.
• prefer-html-body: Set to true to use the HTML body from email, if available, otherwise use plain text.

smtp.auth Section

This section controls SMTP authentication. When enabled, clients must authenticate before they can send an email.

• enabled: Set to true to require client authentication. If false, the server will operate as an open relay (though still subject to policy restrictions).
• user-database: The absolute path to the user database file. This file contains the usernames and their corresponding bcrypt-hashed passwords.

User Database File

The user database is a simple text file where each line represents a user in the format username:bcrypt_hash. Lines starting with # are treated as comments and are ignored.

Example users.db:

# User for our monitoring system
monitoring:$2a$10$abcdefghijklmnopqrstuv.abcdefghijklmnopqrstuv.abcdefghij

You can generate a bcrypt hash for a password using various tools. For example, with htpasswd (from Apache utils):

# Install apache2-utils if you don't have it
# Debian/Ubuntu: sudo apt-get install apache2-utils
# CentOS/RHEL:   sudo dnf install httpd-tools

htpasswd -B -n newuser

# The command will ask you for a password and will output something like:
# newuser:$2y$10$P.gL4v2hergM3sU3B5t25eRzWpE7YzzHSHx4xX3jXyS.i3cZz3n.O
#
# Copy the output into your users.db file.

smtp.policies Section

Policies allow you to control which emails are accepted based on sender (from) and recipient (to) addresses. This is useful for preventing unauthorized use.

Rules are processed in this order:

1. If an address matches a pattern in the deny list, it is rejected.
2. If an address matches a pattern in the allow list, it is accepted.
3. If an address matches no patterns, the default-action is applied.

Fields:

• default-action: Determines the behavior for an address that does not match any allow or deny patterns. Can be allow or deny.
• allow: A list of glob patterns. Addresses matching these patterns are allowed.
• deny: A list of glob patterns. Addresses matching these patterns are denied.

Glob Patterns: The matching is case-insensitive.

• *: Matches any sequence of characters.
• ?: Matches any single character.

Example:

• *@example.com matches any user at example.com.
• user-?@domain.com matches user-1@domain.com, user-a@domain.com, etc.

slack Section

This section configures the Slack integration.

• token: The Slack Bot User OAuth Token for your Slack app. It usually starts with xoxb-. This is a required field. It can be set via the SLACK_TOKEN environment variable or a file specified with --slack.token-file.

Command-Line Flags

Flags can be used to override settings from the configuration file.

Flag	Shorthand	Description	Default
--config-file		The path to look for the configuration file (config.yaml).	./config.yaml
--log-level		The log level to use.	INFO
--smtp.listen-addr		Listen address for the SMTP server.	localhost:25
--smtp.auth.enabled	-a	Enable SMTP authentication.	false
--smtp.auth.user-database		Path to the user database file.
--smtp.prefer-html-body		Use HTML from email, if available, otherwise use plain text	true
--slack.token-file		The path to a file containing the Slack token.
--help	-h	Prints this help message.
--version	-V	Prints the version.

Environment Variables

A few key settings can be provided via environment variables for convenience, especially in containerized environments.

Variable	Description
LOG_LEVEL	Overrides the log-level configuration.
SLACK_TOKEN	Overrides the slack.token configuration. This is the most common way to provide the token securely.