This project provides a simple mail relay server as a Docker image to forward mails to another mail server. It can be useful in these situations:

• Collect mails from several services and forward them to a central mail server. This way, credentials of the central mail server can be kept private and don't need to be shared with the individual services.

• The existing mail server does not provide any interface to interact with an existing service. For example, a service could only support sending mail with STARTTLS while an existing mail server only supports TLS or no encryption.

Currently, mail can be received over plain SMTP or over STARTTLS. The target server can be reached over plain SMTP, STARTTLS or TLS.

Prebuilt images are available on DockerHub. Run the image with Docker and provide the configuration as environment variables to the Docker container.

Examples:

• Create a mail relay server which accepts mail without any encryption:

  docker run \
    --detach \
    --rm \
    --name mail_relay \
    -p 8025:25 \
    -e MAILPROXY_THIS_PORT="25" \
    -e MAILPROXY_THIS_ENCRYPTION="NONE" \
    -e MAILPROXY_DEST_SERVER_NAME="smtp.gmail.com" \
    -e MAILPROXY_DEST_PORT="587" \
    -e MAILPROXY_DEST_ENCRYPTION="STARTTLS" \
    -e MAILPROXY_DEST_USERNAME="<username>" \
    -e MAILPROXY_DEST_PASSWORD="<password>" \
    docker.io/sciapp/mail_relay:latest

  This will accept mails sent with plain SMTP on localhost port 8025 and forward them to Google Mail over STARTTLS. Instead of STARTTLS, one could also choose TLS over port 465.

• Create a mail relay server which accepts mail only over STARTTLS but without any authentication:

  docker run \
    --detach \
    --rm \
    --name mail_relay \
    -p 8587:587 \
    -v `pwd`/cert:/etc/ssl/custom_certs:ro
    -e MAILPROXY_THIS_PORT="587" \
    -e MAILPROXY_THIS_ENCRYPTION="STARTTLS" \
    -e MAILPROXY_THIS_TLS_KEY_FILEPATH="/etc/ssl/custom_certs/my.key" \
    -e MAILPROXY_THIS_TLS_CERT_FILEPATH="/etc/ssl/custom_certs/my.pem" \
    -e MAILPROXY_DEST_SERVER_NAME="smtp.gmail.com" \
    -e MAILPROXY_DEST_PORT="587" \
    -e MAILPROXY_DEST_ENCRYPTION="STARTTLS" \
    -e MAILPROXY_DEST_USERNAME="<username>" \
    -e MAILPROXY_DEST_PASSWORD="<password>" \
    docker.io/sciapp/mail_relay:latest

  This needs a valid SSL certificate bundle to work. In this example, the certificate files are stored in a sub directory cert in the current working directory.

• Create a mail relay server which accepts mail only over STARTTLS and with correct credentials (LOGIN or PLAIN method):

  docker run \
    --detach \
    --rm \
    --name mail_relay \
    -p 8587:587 \
    -v `pwd`/cert:/etc/ssl/custom_certs:ro
    -e MAILPROXY_THIS_PORT="587" \
    -e MAILPROXY_THIS_ENCRYPTION="STARTTLS" \
    -e MAILPROXY_THIS_TLS_KEY_FILEPATH="/etc/ssl/custom_certs/my.key" \
    -e MAILPROXY_THIS_TLS_CERT_FILEPATH="/etc/ssl/custom_certs/my.pem" \
    -e MAILPROXY_THIS_USERNAME="<username>" \
    -e MAILPROXY_THIS_PASSWORD="<password>" \
    -e MAILPROXY_DEST_SERVER_NAME="smtp.gmail.com" \
    -e MAILPROXY_DEST_PORT="587" \
    -e MAILPROXY_DEST_ENCRYPTION="STARTTLS" \
    -e MAILPROXY_DEST_USERNAME="<username>" \
    -e MAILPROXY_DEST_PASSWORD="<password>" \
    docker.io/sciapp/mail_relay:latest

For debugging purposes, the environment variable MAILPROXY_DEBUG can be set to 1 to enable debug logging.