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The basis of a maintainable and stable software system is the ability to easily unit test the system in an automated way using testing frameworks such as JUnit. Unfortunately, verifying correct delivery of a system's email messages and validating content in an automated fashion is not a trivial task due to the complexity of typical email servers. Such testing is possible but tricky and may involve complex administration tasks or integration with proprietary message stores—tasks not conducive to successful automated testing.
I had been thinking about this problem for some time when I came up with the idea of writing my own SMTP (Simple Mail Transfer Protocol) server designed from the ground up to be used solely as an aid to automated testing. The idea was to create a server that could respond appropriately to SMTP commands, but, rather than attempting to deliver email messages to the receiver or forward to another server as appropriate, it would simply cache the received headers and message body for later extraction and verification by a test client.
One of the main attractions of using Java for network-based applications is access to a range of quality networking libraries
provided as standard in the Java development environment. Complex yet robust networking software can be written in a few lines
of code. In Java, the
javax.mail package may be used to communicate with both email servers for the delivery of SMTP-based email messages, and message stores
using POP (Post Office Protocol) or IMAP (Internet Message Access Protocol). This article will concentrate on the former—SMTP
to send email messages—rather than message-store protocols that a user agent would use to extract messages from the store.
The basic requirements for such a "dummy" server are:
This article explains how to write this simple email server; see Resources to download its code.
Before we start writing an SMTP server, an overview of the standards documents that we will use in building the dummy SMTP server is in order.
Most commercial email servers in the world today support the SMTP protocol, originally defined in RFC (Request for Comments) 821 in August 1982. As an historic side note, the word simple in Simple Mail Transfer Protocol is derived from the fact that SMTP itself was based on an earlier more complex protocol definition named, unsurprisingly, the Mail Transfer Protocol (MTP). RFC 821 itself is currently being superceded by the draft standard RFC 2821, published in April 2001. This new draft standard adds no new functionality or updates to existing functionality, so, for the remainder of this article, when referring to SMTP, I mean the protocol as defined in RFC 2821. Since we will not be implementing anything more complex than the basic set of SMTP commands, for all intents and purposes, use of this later draft standard should not affect our solution.
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