The hardware basics
No matter the chip inside, all e-mail servers need plentiful memory and disk resources. Given a choice between a faster processor and more RAM, with e-mail servers you should always opt for more memory.
As for disk requirements, you should have at least a giga- byte of disk room for every 1,000 users. You can get by with less, but you'll be asking for trouble.
Needless to say, all your e-mail servers also should be dedicated servers. Big-time mail hosters also find that it's worth their time to keep a dedicated domain name services (DNS) server on tap. Ideally, a DNS server should be only a Fast Ethernet link away from the mail servers.
iPlanet messaging server
If the name doesn't sound familiar, that might be because you've only known iPlanet Messaging Server (IMS) 5.0 from its two immediate ancestors: Sun Internet Mail Server (SIMS) 4.0 and Netscape Messaging Server (NMS) 4.15. While there have been earlier versions of it, IMS is the first completely merged version.
IPlanet Messaging Server's ancestors have a reputation for being good, solid mail servers with high reliability. Mail administrators like it because, combined with iPlanet Directory Server and its most common deployment on Sparc systems running Solaris, it gives them a familiar suite for the most demanding mail jobs. While IMS currently supports only Solaris, a forthcoming edition will also support Red Hat Linux.
But that new version, out since November, does combine two very different feature sets. The core mail transfer agent, for example, is from Innosoft (by way of SIMS). But the message store is derived mostly from NMS. From our hands-on experience and ISP reports, that hybrid blend mail server is, nevertheless, flawless. But cautious mail administrators might want to wait until IMS 5.1 arrives shortly. While that version will primarily offer better localisation features, it also will include bug fixes.
