Berkeley sendmail, from Eric Allamn and company at Sendmail, Inc., has the latest source code and release notes for the most widely used UNIX mail daemon. Also refer to the sendmail FAQ for more information.

smrsh is a restricted shell utility that provides the ability to specify, through a configuration, an explicit list of executable programs. When used in conjunction with sendmail, smrsh effectively limits sendmail's scope of program execution to only those programs specified in smrsh's configuration.

Exim is an MTA developed at the University of Cambridge for use on UNIX systems. Exim is more or less a drop in replacement for sendmail with some different features like built-in filtering, mutiple different run users, frozen and thawed messages, and more granular message delivery timing (local vs remote). Exim does not handle the variety of message and header formats available with sendmail, though.

Postfix, formerly known as VMailer, is Wietse Venema's attempt to provide an alternative to the widely-used sendmail. Postfix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Weitse has a long and respected history in the field of UNIX security.

Qmail is a freeware sendmail replacement written by Dan Bernstein. Bernstein claims that qmail is a secure, reliable, efficient, simple message transfer agent. It is meant as a replacement for the entire sendmail-binmail system on typical Internet-connected UNIX hosts. Commercial support is also available.

Smail is another freeware sendmail replacement originally written by Ronald S. and Landon Curt Noll and now taken over by a group of developers. Smail comes distributed with many versions of linux.

Zmailer is a multi-process mailer, using two daemon processes to manipulate messages. One of these processes is a router, and makes all decisions about what should happen to a message. The other daemon is a message queue manager, used to schedule delivery of messages.

Sun Solstice Internet Mail is based on SMTP and MIME and supports IMAP4, POP3, and SMTP. A 30 day free trial server software and free client software are available for the public use.

The Netscape Messaging Server is both an MTA and an LDA rolled into one product. The messaging server runs on both UNIX and NT platforms, and replaces sendmail on a client UNIX machine. Both IMAP and POP are supported as well as MIME attachements. Like all netscape products, the messaging server has the standard netscape front end GUI.

IMAP4 (Internet Message Access Protocol) information is available from the University of Washington. This information includes an overview of what IMAP is, technical specs and a comparison to POP, and a listing of IMAP clients and servers.

Qualcomm's popper POP3 server and pop password server allow PC users to change their UNIX password and retrieve their mail from a UNIX host for storage on their own PC.

PMDF from Innosoft International, Inc. is a commercial product which integrates different mail systems into a central mailhub. It works with PC mail systems, POP, IMAP, X.400, SMTP, MIME, pagers, printers, UUCP, mailing lists, and more. The server software runs on Solaris, Digital UNIX, and OpenVMS.

Brent Chapman's majordomo is a freeware mailing list package written in perl5. Majordomo allows for ease of list administration by utilizing email and a simple cleartext password authentication of the administrator. List members are also allowed certain functions such as (un)subscription and retrieval of list information or past list digests. Each list has it's own configuration file that details what privileges users will have and how messages are rewritten. Majordomo does no actual delivery but relies on an MTA such as sendmail.

Smartlist is another automated mailing list management software package. Smartlist is built on top of the procmail mail sorting package and can intelligently auto-remove list members with bouncing addresses. Like majordomo, it allows for per-list customizations and some form of moderation and archiving.

ListProc is a commercial product produced by CREN that is based on the free package Listproc. CREN's ListProc provides extensive list-control and list-configuration options to list owners, moderators, and managers through easily used commands. List archives may also be set up to be accessible through the web.

The mail filtering FAQ gives examples of how to filter and sort mail with various different UNIX packages such as procmail, filter, and mailagent.

Procmail is a C program written by Stephen R. van den Berg that can act as a mail delivery agent and can automatically process and sort mail as you receive it or after it is already in a message folder. Procmail recipes can be a complex mixture of header information used to separate mail into different folders.

Filter is a mail sorting program that comes with the elm software. The filter program uses a simple language of "if then do" tests and allows testing before implications. Filter is invoked through the .forward file as procmail can be.

Mailagent is automatic mail-processing tool written in perl, written and maintained by Raphael Manfredi. Filtering rules are specified using lex-style rules, i.e. they have a set of patterns in the left hand-side and a set of actions within braces on the right hand-side.

Rmail is the default mail package distributed with emacs. Rmail is a major mode within emacs and stores all its mail folders in babyl format.

Vm is a highly extensible emacs add on mail package written by Kyle Jones that stores mail in regular UNIX mail format. Vm also has hooks for virtual folders and various different kinds of sorting. Vm supports mime headers and with another add on package, PGP encryption.

The best references on MH and its interfaces are available from the MH & xmh book from O'Reilly & Associates, the mh FAQ, and the exmh mailing list archive. MH is Rand's Mail Handler and stores each mail message as a separate UNIX mail file. MH does it's folder storage in a hierarchical directory structure.

Elm is a curses/text based mail reader and package of tools to manipulate mail messages. Elm comes with the MUA binary itself, the program frm that checks for new mail, the program printmail which paginate mail files for clean printing, and the filter program which sorts mail based on specific criteria. More information can be gained from the (slightly outdated) elm FAQ. Newer beta versions of the elm software are now out but not listed at the UUNet site.

Though written from scratch, mutt's initial interface was based largely on the ELM mail client. To a large extent, Mutt is still very ELM-like in presentation of information in menus. As development progressed, features found in other popular clients such as PINE and MUSH have been added, the result being a hybrid, or ``mutt.'' At present, it most closely resembles the SLRN news client.

Pine is another curses/text based mail reader that came about from the developers of elm. Pine includes an on-screen help menu and key binding listing as well as allowing one to post to newsgroups as well as send mail. Pine is MIME compatible and can use the program fetchmail as a popserver. The pine FAQ provides more information. Also visit the Pine Information Center at the University of Washington.

Ishmail is a commercial X based mail reader developed by HAL Software Systems. Ishamil supports IMAP4, POP3, and SMTP and is available for SunOS, Solaris, AIX, HP-UX, Digital UNIX, Linux, IRIX, and BSDI. Ishmail allows for individual mail box alerts and bitmap maskings for incoming mail as well as general filtering rules allowing direct delivery to said mailboxes. Ishmail comes with it's own text editor but can be customized to use your favorite editor.

Allowing controlled SMTP relaying in Sendmail 8.9 and anti-spam configuration control cover the features and rulesets used in sendmail that control relaying. This documentation is invaluable for any site running sendmail.

email spamming countermeasures from CIAC talks mostly about filtering on the MTA based on domains, IP addresses, and email addresses. They also list some do's and don't's for when you do receive spam.

Fight Spam on the Internet! is a campaign to fight spam. The goal is to outlaw spam entirely, but the site also has some basic spam FAQs, suggestions on how to market on the net WITHOUT spamming, how to create a good spam policy, and lists of rogue sites.

CAUCE, the Coalition Against Unsolicited Commercial Email, is an ad hoc, all volunteer organization, created by Netizens to advocate for a legislative solution to the problem of UCE. The site is designed to provide information about the problems of junk email, some proposed solutions, and to provide resources for the Net Community to make informed choices about the issues surrounding junk email.

MAPS, the Mail Abuse Prevention System, is a California nonprofit company whose goal is to stop the Internet's email system from being abused by spammers. Its principal means of doing this is by educating and encouraging ISP's to enforce strong terms and conditions prohibiting their customers from engaging in abusive email practices. This site includes Paul Vixie's RBL, the Realtime Black Hole list. The RBL is a subscription system for creating intentional network outages for the purpose of limiting the transport of known-to-be-unwanted mass email.

RFC 1725 details the technical specifications of version 3 of the the post office protocol (POP3).

RFC 1734 details the POP3 authentication extensions.

RFC 821details the simple mail transport protocol (SMTP).

RFC 822 details the standard format of Internet messages.

MIME (Multipurpose Internet Mail Extensions) builds on older email standards that only allowed ASCII text transmissions by standardizing additional fields for mail message headers that describe new types of content and organization for messages. MIME allows messages to contain multiple objects in a single message, text having unlimited line length or overall length, character sets other than ASCII, multi-font messages, binary or application specific files, and images, audio, video and multi-media messages. The MIME FAQ offers more information on the protocol and the programs which use it.

PGP is an RSA derived public key encryption mechanism developed by Phil Zimmerman. Though PGP is most often associated with email encryption it can also be used for general file encryption. To make mail encryption easier on the end user, several mail programs have hooks to incorporate PGP directly into the software. BAL's public PGP key server is a good place to find keys belonging to many people, including those who work in the security and software development fields.

Mailcrypt is an add on emacs lisp package which provides a simple interface to message encryption with PGP.

CMU's email web resources page contains useful info about standards, clients, servers, gateways, robots, email services, how-tos and FAQs.