SMTP Proxy Server for Forwarding Emails (w/GPG)

This project implements an SMTP proxy server that allows clients to send emails using a single shared email address, but different credentials. This can be particularly useful if you cannot or prefer not to have multiple email addresses for such clients, but you still want them to have separate and distinct credentials for security purposes. For example, when different services are sending mail to an external mail server and sharing a mailbox there.

The SMTP proxy intercepts email traffic from apps, checks credentials for each one, modifies the "From" header to ensure the email is sent from a common address, and forwards the email to the desired recipients via a specified SMTP server.

Additionally, if configured to do so, this proxy now automatically identifies users who have published GPG keys on keyservers and ensures end-to-end encryption for the messages they should receive. This feature makes email transmission secure by encrypting emails for recipients with published GPG keys, adding an important layer of confidentiality and security to your communication.

Configuration hot-reloading is supported and will happen on changes to the configuration, so be sure to copy the file elsewhere if you think you could save malformed YAML.

See config.example.yml for a sample configuration (rename to config.yml and customize to use).

Use Case

This SMTP proxy server is ideal for scenarios where:

• Multiple applications on a server need to send emails.
• You do not want to or cannot configure separate email addresses for each app.
• You want to ensure all outgoing emails are sent from a single, unified email address.
• You want to avoid sharing credentils between applications.
• You are managing a server with limited email configuration capabilities.

Or, most likely, your applications do not support GPG and you want a middleware for encrypting outbound emails. (Yes, this project has grown a bit out of the original scope).

Requirements

• Python 3.7+
• Required Python libraries:
  • aiosmtpd
  • PyYAML
  • watchdog
  • python-gnupg
  • requests
  • jsonschema

Install the required dependencies using pip:

pip install -r requirements.txt

An example systemd process is provided.