Smail-3 is an implementation of an Electronic Mail Transport Agent (MTA).
Version: 220.127.116.11Smail-3 is an implementation of an Electronic Mail Transport Agent (MTA), i.e. a program used for sending and receiving electronic mail, for Unix-like (POSIX compatible) systems.
Operating System: Linux
Smail's job is to accept mail messages from local files or programs running on the local machine (such as any mail user agent), or from remote hosts by way of network protocols such as SMTP or UUCP, and deliver those messages to the appropriate destinations, be they to remote hosts or to files or programs on the local machine. Smail-3 can also gateway e-mail traffic bi-directionally between disparate underlying mail transport systems, for example between SMTP and UUCP.
Smail is a sendmail-like agent for transmitting mails, but is more simple and user-friendly.
In fact Smail-3 is primarily known as one of the most simple to understand and easy to configure modern mail transfer agents. Its out-of-the box configuration is quite suitable for almost any small to medium sized domain and it can be used with little or no local customization.
Smail-3 has also proven to be a capable MTA that can meet the demanding needs of medium sized businesses and ISPs. Even a modest old P-II PC-class server running with Smail and a decent mail store such as Cyrus IMAP can handle all of the e-mail needs for tens of thousands of users provided it has a decent and fast disk subsystem and enough RAM. When running on enterprise-class hardware, Smail will provide enterprise-class service.
Smail-3 has relatively strong security (at least for a big monolithic program that often runs with superuser privileges) as it was implemented using many secure and safe coding practices that avoid most of the common pitfalls of similar large programs written in C. As far as I can tell it has only ever been the subject of one security advisory. Smail also has very good defenses against network abuse such as denial of service attacks. As part of those defenses Smail also offers extensive access control features and filtering capabilities that can be used to block unwanted junk e-mail. These features include many types of basic access control lists for implementing policies for SMTP clients, the ability to use DNS based access lists (both IP and hostname based) for various SMTP parameters, as well as full regular expression (using PCRE) matching on content with separate passes for message headers and message bodies. Smail can also do various DNS consistency checks on client mailers and the SMTP envelope sender address (including doing anti-spoof protection for sender addresses in locally deliverable domains).
Note that Smail-3 is not intended to be a user interface for reading and submitting mail - it is instead the interface to the network and mail delivery systems that those mail readers use. Smail is also not a mail store and does not provide services such as POP or IMAP - but you can use Smail with something like Cyrus IMAP for that purpose.