Mailific is a library for building SMTP listeners in Java, with an implementation built on Netty. In addition to the basic SMTP dialog, it supports important extensions including STARTTLS, SMTPUTF8, and AUTH.
The library I've used for this purpose for many years is subethasmtp. But Subetha isn't actively maintained. It lacks support for internationalized email addresses. I prototyped adding IEA to subethasmtp, but in the process, I kept wanting to reorganize the whole project. So I decided to start from scratch, instead.
Although functional, it is still in an early stage and has not seen much usage. Of particular concern, it does not yet do much checking for standards compliance. (It does attempt to be standard compliant itself.)
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Interoperate with RFC compliant senders.
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Clean separation of SMTP logic from networking implementation.
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Minimal dependencies (other than Netty, as the only currently implemented networking stack). No dependency on JavaMail.
- Default behavior should be to not tolerate non-RFC compliant behavior.
- Easy ways to override for more tolerance.
- Add libraries for authentication checking: SPF, DKIM, DMARC.
- Add sending support (as a separate library) for relaying.
- SAX-like message-parsing library (e.g., receive headers and body parts as events)
- Separate project that adds Spring integration
- Release integration test suite as a separate project
Note that in future versions (time allowing), further compliance checks will be added and will be on by default. So if you depend on lenient behavior in the current version, you should read the change log and be prepared to to update your code to specify the lenient behavior.
- No soft shutdown.
This section will walk through some progressively more complicated tasks.
You can start up a simple demo server by just running java -jar <mailific jar>. By default it will
listen on port 2525. Pass -h to see some options. This server just accepts any message you send in
and discards it.
To start up a listener in your own app, you need to construct a NettySmtpServer (the included concrete implementation of SmtpServer) and call start() on it. Let's walk through what you need for that.
First, you need a MailObjectFactory. We'll talk through MailObjects in detail below, under Processing Messages. To start with, you can just construct a BaseMailObjectFactory, which creates MailObjects that simply discard all message data.
Next, you need a ServerConfig. You can start from scratch using the ServerConfig.builder(), but for now, call Main.defaultServerConfigBuilder(MailObjectFactory), which populates all the builder fields with defaults that you can override.
Finally, create and start a server with new NettySmtpServer(serverConfig).start().
You can shut id down with SmtpServer.shutdown(). Be warned that shutdown is not graceful: it kills
any sessions in flight.
A server that just discards messages is not very useful (except maybe in a test environment). To
actually do something with incoming mail, you need to implement MailObject. The simplest way is to
extend InMemoryMailObject and do your processing in processFinished(byte[]).
Once you've got your MailObject class, implement a MailObjectFactory that returns it. You can pass that into Main.defaultServerConfigBuilder() when building your server.
If you want to modify the handling of an SMTP command, you can either create your own implementation of CommandHandler, extend BaseHandler, or extend the standard handler for the verb you want to modify. Let's say for example, that you want your server's response to EHLO to include a random joke. You could create a class called JokeEhlo and extend Ehlo. The only method you would need to override is getGreeting(), which would need to return your random joke.
OK, now you've got JokeEhlo, what do you do with it? You want to include it in the collection of CommandHandlers in the ServerConfig. An easy way to override just one or two of the standard CommandHandlers is to start with Main.baseCommandHandlers(). It returns a Map with all the default command implementations, keyed by verb. To replace the standard Ehlo command with an instance of yours, call myCommandMap.put(jokeEhlo.verb(), jokeEhlo). Then, call serverConfigBuilder.withCommandHandlers(myCommandMap).
Notice that you're passing a single instance of your JokeEhlo into the ServerConfig -- not, say, a JokeEhloFactory. CommandHandlers have to be stateless, reentrant Singletons, because the same instance is going to be used concurrently for all incoming SMTP sessions.
The library comes with implementations for most of the standard SMTP commands. I haven't gotten around to VRFY and EXPN yet.
You can turn on StartTLS (as describe in RFC 2487) by supplying the ServerConfig with a TLS cert-chain file (in PEM format) and the key for that cert in pkcs8 format. If the key is encrypted, you must also supply the password. As of now, those are the only formats provided.
The library includes support for several SMTP extensions, and it's fairly easy to add your own. RFC 1425 doesn't really limit what effects an SMTP extension can have, and some of them alter server behavior drastically. So it's impossible to write an extension framework that anticipates any conceivable extension. Hopefully the Extension interface will get you pretty far.
The method Main.harmlessExtensions() returns a collection of extensions that you might as well include, since they "just work" and improve the server's capabilities. Look in the net.mailific.server.extension package to see what else is available, and to see some code examples.
In case you're poking around in there, be aware that StartTls is the one supplied extension that isn't implemented purely as an extension: there's code in the server implementation that interacts with it.
Extend Connect, and override the method shouldAllow(SmtpSession). You can get the remote IP from SmtpSession.getRemoteAddress().
There are two ways to go about this. If you are extending BaseMailObject, you probably want to override mailFrom() and offerRecipient() to return a non-2xx reply code for blocked senders/recipients. You could also create your own Mail and/or Rcpt CommandHandlers.
The library supports SMTP Auth (as detailed in RFC 4954) using either the PLAIN or LOGIN SASL mechanism. To turn this on, you must
- implement the AuthCheck interface,
- use your Authcheck to construct a PlainMechanism and/or LoginMechanism,
- use your mechanisms to construct an Auth extension, and
- include the Auth extension in the ServerConfig.
There's an example of all this in the Main.getTestAuthExtension method.
You may note that there are lots of unit tests but no integration tests. I'm using a separate project to do integration tests. I hope to release that soon, as well.