Fake SMTP Server

Simple SMTP Server which stores all received emails in an in-memory database and renders the emails in a web interface

Introduction

The Fake SMTP Server is a simple SMTP server which is designed for development purposes. The server collects all received emails, stores the emails in an in-memory database and provides access to the emails via a web interface.

There is no POP3 or IMAP interface included by intention. The basic idea of this software is that it is used during development to configure the server as target mail server. Instead of sending the emails to a real SMTP server which would forward the mails to the target recipient or return with mail undelivery for test email addresses (e.g. @example.com) the server just accepts all mails, stores them in the database so that they can be rendered in the UI. This allows you to use any test mail address and check the sent email in the web application of the Fake SMTP Server.

The server store a configurable maximum number of emails. If the maximum number of emails is exceeded old emails will be deleted to avoid that the system consumes too much memory.

The server is also provided as docker image on docker hub gessnerfl/fake-smtp-server. To change configuration parameters the corresponding configuration values have to be specified as environment variables for the docker container. For details check the Spring Boot (http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-external-config) and docker documentation (https://docs.docker.com/engine/reference/run/#env-environment-variables).

Running Fake SMTP Server locally

Note

Starting with version 2.2.0 Java 21 is required to run Fake SMTP Server.

Note

Starting with version 2.0.0 Java 17 is required to run Fake SMTP Server.

Run from released JAR files

1. Download the latest fake-smtp-server-<version>.jar from https://github.com/gessnerfl/fake-smtp-server/releases/latest
2. Copy the file into the desired target folder
3. Execute the following command from the folder where the JAR file is located:

java -jar fake-smtp-server-<version>.jar

Run from sources

In order to run this application locally from sources, execute:

./gradlew bootRun

Afterwards, the web interface is be availabe under http://localhost:8080.

Configuration

As the application is based on Spring Boot the same rules applies to the configuration as described in the Spring Boot Documentation (http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-external-config).

The configuration file application.yaml can be placed next to the application jar, in a sub-directory config or in any other location when specifying the location with the parameter -Dspring.config.location=<path to config file>.

All configuration parameters can also be passed as environment variables using uppercase characters and underscores as separators such as SERVER_PORT or MANAGEMENT_SERVER_PORT.

The following paragraphs describe the application specific resp. pre-defined configuration parameters.

Fake SMTP Server

The following snippet shows the configuration of a fake smtp server with its default values.

fakesmtp:
  #The SMTP Server Port used by the Fake SMTP Server
  port: 8025

  #The binding address of the Fake SMTP Server; Bound to all interfaces by default / no value
  bindAddress: 127.0.0.1

  #List of recipient addresses which should be blocked/rejected
  blockedRecipientAddresses:
    - blocked@example.com
    - foo@eample.com

  #List of sender email addresses to ignore, as a comma-separated list of regex expressions.
  filteredEmailRegexList: john@doe\\.com,.*@google\\.com ; empty by default

  #Optional configuration option to specify the maximum allowed message size. The size can be 
  #defined using Spring Boot DataSize value type - https://docs.spring.io/spring-boot/docs/2.1.9.RELEASE/reference/html/boot-features-external-config.html#boot-features-external-config-conversion-datasize.
  #Default: no limit
  maxMessageSize: 10MB

  #Configure if TLS is required to connect to the SMTP server. Defaults to false. See TLS section below
  requireTLS: false

  #When set to true emails will be forwarded to a configured target email system. Therefore
  #the spring boot mail system needs to be configured. See also 
  # https://docs.spring.io/spring-boot/docs/current/reference/html/spring-boot-features.html#boot-features-email
  forwardEmails: false

Authentication

Optionally authentication can be turned on. Configuring authentication does not mean the authentication is enforced. It just allows you to test PLAIN and LOGIN SMTP Authentication against the server instance.

fakesmtp:
  authentication:
    #Username of the client to be authenticated
    username: myuser
    #Password of the client to be authenticated
    password: mysecretpassword 

TLS

Optionally TLS can be activated. To configure TLS support, a trust store needs to be provided containing the TLS certificate used by the FakeSMTP Server.

fakesmtp:
  # true when TLS is mandatory otherwise TLS is optional
  requireTLS: true
  #configuration of the truststore to enable support for TLS.
  tlsKeystore:
    location: /path/to/truststore.p12
    password: changeit
    type: PKCS12 # or JKS

Data Retention Settings

Emails

To keep memory resources under control, there is a parallel process that deletes the oldest emails considering the maximum number of emails to retain and the time span to periodically recheck this maximum number of emails, controlling also the initial time to wait to start this parallel process. The default values are:

• maxNumberEmails: 100
• fixedDelay: 300000 # 5 minutes
• initialDelay: 60000 # 1 minute

fakesmtp:
  persistence:
    maxNumberEmails:
      emails:
        #max numbers of most recent emails to retain and not deleted by the parallel process
        maxNumberEmails: 10
        # configuration settings of the background process (timer) responsible to delete the oldest emails
        emailDataRetentionTimer:
          #each 5 minutes from 'initialDelay' (see below), the parallel process will check if the deletion is necessary
          fixedDelay: 300000
          #each 'initialDelay' (see above)  after 2 minutes from the start, the parallel process will start checking if the deletion is necessary
          initialDelay: 120000

Web UI

The following snippet shows the pre-defined web application configuration

#Port of the web interface
server:
  port: 8080

#Port of the http management api
management:
  server:
    port: 8081 

Web UI Authentication

You can optionally enable Basic Authentication for the web interface and REST API. When authentication is enabled, users will need to log in to access the web interface and API endpoints.

To enable authentication, set the username and password in the application.yml file:

fakesmtp:
  webapp:
    authentication:
      # Username for web UI and API authentication
      username: admin
      # Password for web UI and API authentication
      password: password

You can also set these values using environment variables:

FAKESMTP_WEBAPP_AUTH_USERNAME=admin
FAKESMTP_WEBAPP_AUTH_PASSWORD=password

If both username and password are not set, authentication will be disabled and the web interface and API endpoints will be accessible without authentication.

When authentication is enabled:

• The web interface will show a custom login form
• API endpoints will require Basic Authentication
• A logout button will be available in the navigation bar
• The following endpoints are protected and require authentication:
  • Main web UI routes (/, /emails/**)
  • API endpoints (/api/**) except for /api/meta-data
• The following endpoints remain public and do not require authentication:
  • /api/meta-data (provides application metadata including authentication status)
  • Swagger UI (/swagger-ui.html, /swagger-ui/**, /v3/api-docs/**)
  • Actuator endpoints (/actuator/**)
  • H2 console (/h2-console/**)
  • Static resources (/static/**, /css/**, /js/**, /images/**, /webjars/**, /favicon.ico)

Note

The Web UI authentication is separate from the SMTP server authentication. The SMTP server authentication is configured under fakesmtp.authentication and is used for authenticating SMTP clients, while the Web UI authentication is configured under fakesmtp.webapp.authentication and is used for authenticating users accessing the web interface and API endpoints.

Real-Time Email Notifications

The Fake SMTP Server supports real-time email notifications using Server-Sent Events (SSE). This feature eliminates the need for polling and provides instant updates when new emails are received.

How It Works

1. The web interface automatically establishes an SSE connection to the server
2. When a new email is received by the SMTP server, an event is sent to all connected clients
3. The web interface updates the email list in real-time without refreshing the page

Benefits

• Instant Updates: See new emails as soon as they arrive
• Reduced Server Load: Eliminates the need for frequent polling
• Improved User Experience: No delay in seeing new emails
• Efficient Resource Usage: SSE is more efficient than WebSockets for one-way communication

Authentication Considerations

When Web UI Authentication is enabled, the SSE connection is only established after successful login. This ensures that only authenticated users receive real-time notifications.

REST API

Documentation of exposed services is available at:

localhost:8080/swagger-ui.html

Developpment Environment

This requires to have docker installed. If you need to implement a new feature, you will probably need an correct JDK version setup in an environement.

sh/dev

Then, in the dev container started by the command above, you can use various commands. The following commands should be the most common ones:

sh gradlew test
sh gradlew test --tests '*EmailRepositoryIntegration*' --info
sh gradlew build

Run UI & Backend tests

sh/test

Build UI & Backend

sh/build

Run app (UI & Backend)

sh/run

Build & Push a new development docker image

To update/change the development image, update the dev.Dockerfile, dont forget to change the version in the dev-image-tag file and edit the registery if needed.

sh/push-dev-image