There are no specific hardware requirements for mail under Linux.
You'll need some sort of 'transport' software to connect to remote systems, which means either TCP/IP or uucp.
This could mean that you need a modem or ethernet card, depending on your setup. In most cases, you'll want the fastest modem you can afford, i.e. V90 57 600 bps currently. In general, you want to have a 16550 UART on your serial board or built into your modem to handle speeds of above 9600 baud.
If you don't know what that last sentence means, please consult the
comp.dcom.modems group or the various fine modem and serial
communications FAQs and periodic postings on USENET.
Well, the problem is here. Which mail software will you choose ?
There is currently qmail, smail, vmail and sendmail.
Each has its own features, but the better compromise is qmail, for high security (even is vmail is more secure), high speed (even is smail is faster for local uses) and ease of configuration.
Of course, feel free to choose any mail software, informations provided here shall only help you in your choice.
Sendmail can be nice for many sites with complicated options, but I think its configuration is too hard for beginners while it is not very secure or very fast, so there is only a really outdated sendmail section in this HOWTO.
If you know what you're doing, choose sendmail (and you shouldn't be reading this HOWTO !); otherwise I generally recommend qmail.