Microsoft Exchange and other mail servers / mail transport agents

Mail Servers / Mail Transport Agents

	Exchange is typical Microsoftware, a Swiss Army knife that's supposed to handle all your messaging needs. But after using the fold-out scissors, the pop-out screwdriver, the collapsible corkscrew, and so on, you often wish for a nice big set of shears, a screwdriver with a good handle, a corkscrew with leverage... the best of each kind of tool. Or just the tool you actually need, without having to lug around that bulky thing with the built-in spoon. Exchange was originally designed for local Windows-only mail; the internet connectivity was tacked on when Microsoft discovered that wasn't just a fad. And they charge ridiculous license fees for every user account. It makes no sense when there are so many cheaper and sharper alternatives available, whether you want to run it on Windows, Mac OS, or (ideally) a Unix-like server.
	SendMail is the de facto standard mail transport agent; if you've sent or received more than a few e-mail messages on the internet, SendMail has almost certainly handled some of them. Its configuration files can be daunting (you'll want to use a configuration tool to generate and update them), but since it's open-source, it's free, fully documented, and support can be found everywhere. (SendMail.com offers commercial support, and helps fund SendMail.org) For all Unix-like systems.
	"PostFix attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible..." says the developer, underscoring three areas where the standard-bearer isn't as strong. The design is classic Unix: each specific task is performed by a little program that performs only that task, coordinated by the multitasking operating system, and bound by the clear standards for their behaviour. One nice thing is that its defaults settings are all generally the "right" ones, making it easy to get started with. Free open-source software for Unix-like systems.
	qmail was designed as a fully secure replacement for SendMail (whose fundamental design dates back to when the net was a much less hostile and hectic place). The unclaimed $1000 prize for cracking it suggests that it succeeds at that goal. Furthermore, it's very efficient and reliable. Yahoo uses it for their e-mail service; so did Hotmail, before Microsoft took it over and converted it to use Exchange... introducing service outages and security breaches in the process. Free and published-source (but not true open-source) software for Unix-like systems.
	Exim was written with simplicity in mind (inspired by the old Smail), taking advantage of the fact that e-mail delivery on the internet has become more standardized and predictable now that most server sites are online at all times. Instead it focuses on convenient and flexible configuration, making use of lookup tables, regular expressions, and other fun stuff from Information Theory class. Free open-source software for Unix-like systems.
	Xmail was created by a developer stuck in a Windows environment who needed a high-quality system to handle all the tasks involved with e-mail (sending, delivering, spam filtering, distribution lists, etc.) without the extraneous groupware features bundled into Exchange. (The only "core" e-mail function missing so far is IMAP.) He designed it to be easily ported to Unix-like systems as well, and has released it under an open-source licence.
	Originally developed as part of a joint venture to compete with Microsoft, the Sun ONE Messaging Server is the successor to both Netscape and Sun's earlier mail servers. It can cost some signficant money, but includes all the features a corporate e-mail system is likely to want, including POP and IMAP servers, a web interface, filtering, and LDAP directory integration.
	Lotus Domino is the server component of their ground-breaking Lotus Notes system. In addition to a mail server, it also supports standard web and Lotus' collaboration applications. It runs on Unix-like systems, NT/2000, and various IBM big-iron systems, which means it's damn hard to outgrow.
	Novell has two possible answers to Exchange, depending on your specific needs. GroupWise is an integrated collaboration system, with support for instant messaging, document retrieval, scheduling, and contact management, as well as e-mail. NetMail is an incredibly-scalable standards-based enterprise e-mail server, with calendaring and web access. Both are designed to leverage Novell's universal cross-platform NDS eDirectory (the system Microsoft is trying vainly to duplicate with their Windows-only Active Directory) and the highly-stable, highly-secure Netware operating system. (You can also run them on other popular operating systems). You can see NetMail in action, running the free MyRealBox e-mail service.
	Ipswitch IMail Server (whose WS-FTP client is a popular favourite) replaces most of what you'd do with Exchange (it runs on Windows servers... lots of them), but is far quicker and easier to install and to manage. It can draw user account information from an external database for easier administration, and offers extras such as a web interface, shared calendar management, and server-based spam filtering. It's good for medium-sized organisations that have a little money to spend, but want to spend it wisely.
	Stalker Software offers several mail transport agents which are highly regarded. CommuniGate/Pro is their fully-featured flagship server for enterprise-scale operations, and runs on Unix-like systems, Windows, Mac OS, IBM OS/400, and BeOS. Their just plain CommuniGate server offers a modular set of e-mail, fax, paging, etc. services on Mac OS servers (but supports any standard e-mail client, of course). The free Stalker Internet Mail Server provides just the standard e-mail services (but without skimping on them), also on Mac OS (even a low-end Quadra).
	Mercury is the server from the makers of the popular Pegasus Mail client. It's available in versions that run on Windows and Novell Netware servers. Both versions integrate well with Novell's NDS eDirectory, meaning you can use the same accounts and passwords for both file server and mail server. Among its many features are support for collecting and distributing mail across dial-up connections, which can be useful for small offices without dedicated internet connections.
	MailSite comes in packages ranging from small workgroups to huge enterprises. At the high end (which is where it's most attractive), it has support for external SQL databases (which don't have to run on Windows NT, as MailSite itself does), which is useful for both user accounts and its mailing-list features. It features strong security, web access, and remote administration.
	Alt-N Technologies' MDaemon is an affordable, efficient mail system whose main shortcomings are that it's available only for Windows (95B and later) and that it can't accomodate more than... 50,000 users. As the modest OS requirement suggests, its hardware requirements are fairly modest as well. Both standard and "pro" versions are available, depending on the features you need (e.g. IMAP, multiple domains, virus filtering, calendaring, etc.)
	The Eudora Internet Mail Server is an easy-to-use-and-install, but full-featured server for Mac OS systems (68030 or later), which is now back in the hands of its original developer.
	NetWin DMail is from the developers of the first-class DNews Usenet server, and offers similar configuration tools, including a Windows program that can be used to administer the server remotely regardless of whether it's on a Unix-like or Windows box. It has modules supporting both POP and IMAP, and a mailing list server similar to the venerable ListServ. It can access a wide variety of user databases (or use its own). It's free for up to 5 users, which makes it good for someone who wants a mail server to run with their web server. A web interface (compatible with other mail servers) is available separately.