SubEtha SMTP is a Java library which allows your application to receive SMTP mail with a simple, easy-to-understand API.

This component can be used in almost any kind of email processing application. Hypothetical (and not-so hypothetical) uses include:

• A mailing list manager (see SubEthaMail)
• A mail server that delivers mail to user inboxes
• A mail archiver like The Mail Archive
• An email test harness (see Wiser)
• An email2fax system
• SMTPseudo A filtering forwarding server
• Baton SMTP proxy for one or more backends (rules based on sender/envelope)
• Mireka - Mail server and SMTP proxy with detailed logging, statistics and built-in, fail-fast filters

• Supports minimum SMTP specification described in rfc2821 (4.5.1)
• Supports STARTTLS
• Supports SMTP AUTH
• Supports SMTP over SSL/TLS (via specification of server socket factories)
• Uses builders for concise code and a discoverable API

The code below starts an SMTP server and logs messages to the console:

SMTPServer server = SMTPServer 
  .port(25000) 
  .build();
  
 //start server asynchronously
 server.start();

Do your own thing with a message (from, to and a byte array of data):

SMTPServer server = SMTPServer //
 .port(PORT) //
 .messageHandler(
   (from, to, data) -> 
      System.out.println(
          "message from " + from 
          + " to " + to
          + ":\n" + new String(data, StandardCharsets.UTF_8)))
 .build();
 
server.start();

The builder has a lot of options. This fragment sets a bunch of them:

SMTPServer server = SMTPServer 
  .port(port) 
  .connectionTimeout(1, TimeUnit.MINUTES) 
  .authenticationHandlerFactory(ahf) 
  .backlog(100) 
  .bindAddress(address) 
  .requireTLS() 
  .hideTLS() 
  .hostName("us") 
  .maxMessageSize(10000)
  .maxConnections(20)
  .maxRecipients(20) 
  .messageHandlerFactory(mhf) 
  .executorService(executor)
  .startTlsSocketFactory(sslContext)
  .fromAddressValidator(emailValidator)
  .build();

Use this maven dependency:

<dependency>
    <groupId>com.github.davidmoten</groupId>
    <artifactId>subethasmtp</artifactId>
    <version>VERSION_HERE</version>
</dependency>

SubEthaSMTP was split out of the SubEthaMail mailing list manager because it is a useful standalone component. When we wrote SubEtha, the last thing we wanted to do was write our own SMTP server. In our search for a modular Java SMTP component, we examined:

• Apache JAMES
• JBoss Mail Server, now also defunct Meldware Mail
• Dumbster
• Jsmtpd
• JES
• Green Mail

Since you're reading this page you probably already know what we found: Seven different SMTP implementations without the slightest thought given to reusability. Even Jstmpd, which purports to be a "A Modular Java SMTP Daemon", isn't. Even though JBoss Mail/Meldware Mail is in active development, the team was unintersted in componentization of the SMTP processing portion of their server. GreenMail, which is based on the JAMES code base is best summarized with this blog posting.

During the development of SubEtha's testing harness, we tried out the Dumbster software and found that not only was the API difficult to use, it did it not work properly, the developer has not done any development on it in about a year and it does not work reliably on Mac OS X. With two simple classes we re-implemented it as an included project called Wiser.

We hate reinventing wheels. This should be the LAST FREAKING JAVA SMTP IMPLEMENTATION (Dave Moten: not including forks!).

Engine821.com too did a survey of existing Java SMTP implementations and were unsatisfied... until they found SubEthaSMTP! The code is clean and very well thought out. The changes they made were minor, including...

• Eliminate the embedded /lib directory. Maven correctly handles pulling in all the dependencies and best practices discourage keeping binary artifacts inside version control.
• Update to the latest versions of some of the libraries used.
• Remove some of the IDE metadata files. Your IDE can rercreate whichever ones you need based on your preferences and the Maven POM.
• Make the message handing exceptions be checked. This is possibly controversial, but we thought about it a lot and prefer to have these exceptions show up in the throws clause rather than have them potentially pop-up unexpectedly at run-time.

Dave Moten came across the Engine821 fork and

• fixed tests
• migrated mocking to use Mockito (with apologies but was the most time-efficient way for me to restore the broken tests!)
• set up pom.xml for release under the com.github.davidmoten:subethasmtp artifact
• released to Maven Central
• submitted the changes back to the Engine821.com fork (apart from the groupId change and release changes). Note that review/merge of changes does not seem to be forthcoming.
• added multi-JDK continuous integration using Travis (now using Github Actions)
• added code coverage using coverage.io
• added round trip unit test of STARTTLS
• removed MigBase64 because is complex code without unit tests (even in original source) and Java 8 Base64 is faster
• cleaned up code (made fields private and final where appropriate, remove public keyword from interface methods)
• minor coverage improvements
• required Java 8 (just because of Base64 class at the moment, Java 7 required now because of use in unit test of X509TrustManager)
• converted SMTPServer to be largely immutable and is created with a builder pattern
• adjusted Wiser API to cope with an immutable SMTPServer
• disallowed inheritance of SMTPServer (now final)
• Wiser now created with builder pattern (disallowed inheritance and added accepter builder method)
• used composition instead of inheritance in SmartClient
• used static factory method for SmartClient so that references don't escape the constructor (connect was called from the constructor)
• used explicit character set with InputStreamReader (US_ASCII) in SMTPClient and SmartClient
• used java.util.Optional and Preconditions in SmartClient, SMTPClient and SMTPServer
• added @Override annotations
• added EmailUtils tests
• moved classes that are not part of the public API to internal packages
• added pure SSL support to SMTPServer builder and a roundtrip unit test
• added BDAT support
• added support for receiving inbound SMTP connections via a proxy that uses the PROXY protocol, built on the HAProxy specification found at https://www.haproxy.org/download/2.3/doc/proxy-protocol.txt
• and more, see Releases

Ian McFarland contributed the first codebase to SubEtha Mail. Then, Jon Stevens and Jeff Schnitzer re-wrote most of Ian's code into what we have today. Edouard De Oliveira and Scott Hernandez have also made significant contributions. Dave Moten made changes as mentioned above.

If you have any bug reports, questions or comments about this SubEtha SMTP fork, it's best that you use the GitHub issue tracker to get in touch. Please do not email the authors directly.

For now, we have just focused on implementing just the minimal required aspects of https://rfc.net/rfc2821.html#s4.5.1. We also return SMTP status responses that mimic what Postfix returns.

Thanks to a contribution from Mike Wildpaner, we support the StartTLS specification.

Thanks to a contribution from Marco Trevisan, we support the SMTP AUTH specification.