Development

A comparison of Mail Transfer Agents - Part One

August 23, 2006

This article was contributed by Dan Shearer

For a lot of people the choice of the Mail Transfer Agent is important. The wrong choice can mean lost time and money, lower reliability and increased risk to networks.

Debates over MTAs sometimes last for years, and this article covers the main points that come up over and over. Unfortunately, apart from this article there are no general comparisons of MTA characteristics on the Internet, and even very little benchmarking. The remarks here are personal opinions drawn from readily-verifiable facts and subjective comments drawn from experience. Nearly every MTA has a vociferous and sometimes combative group of supporters, not always including the principal authors of the MTA.

It is easy to see why administrators care about which MTA they use. Large installations require a lot of time spent tuning the MTA, and for any site email is without doubt the most important use of the Internet. End users can get by without a web site or a browser for a little, but without email business stops. And so countless administrators invest time in learning how to tweak their internet mail delivery tool in order to meet their various goals. But which tool should they use when?

Most Internet email seems to be delivered by one of four MTAs:

There are other worthy free MTAs to talk about, such as zmailer and smail3, but since they are not so widely used I decided to omit them. There are some unworthy MTAs too, these I am delighted to omit.

How To Compare MTAs

Each of these four widely-used MTAs have broadly similar features. All of them can handle large amounts of mail; can interact with databases in many formats; have an extensive knowledge of the many SMTP variants in use; are not trivially exploitable; have the source code available in a free manner; have third-party documentation available; and have significant user communities. They even have logos!

There are some assumptions implicit in the rest of this article. If you are looking for a product that presents an administrative interface and performance results similar to Microsoft Exchange or Lotus Notes, this document is not for you. I do not believe either of these products and their aspiring competitors can be classed as MTAs, since they attempt to address dozens or hundreds of other functions besides delivering mail. On the other hand, if you want some guidance for selecting between credible alternatives for an important mail hub, read on.

No MTA can score well in every way of measuring an MTA. The needs of users vary greatly and some criteria are mutually orthogonal. Commonly cited MTA selection criteria are:

Ease of administration
Security
Performance
Long-term viability

Design features decide how much each MTA meets these criteria. But since opinions vary widely there are many equally valid different comparisons. Contradictory examples of these features are:

single configuration file, so everything is in one place
many single-purpose and optional configuration files
minimal and careful syntax
powerful embedded scripting language
maximum code stability
source code contributions regularly incorporated
minimum possible features added

Just about every mail delivery scenario can be met, in one way or another, by all four MTAs. So there is no one right answer.

The rest of part one of this two-part article series is available here, it presents a detailed look at qmail and Postfix. Part two will be featured on next week's LWN.net development page.

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