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. 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. What shall you do with mail software ?
1) Number of sites
Feed more than 100 sites with complicated options for domain name ? Choose sendmail !
Feed less than 100 sites without playing with domain names too much ? Choose smail !
2) Security
Feed more or less than 100 sites but with high security ? Choose qmail !
Feed less than 100 sites but with a standard level of security ? Choose smail !
3) Different ways to receive mail
Feed and being fed by UUCP or FIDO (via ifmail) ? Feed and being fed by POP and internet SMTP ? Choose smail !
Of course, feel free to choose any mail software, informations provided here shall only help you in your choice. Sendmail is perfect for many sites with complicated options, qmail is perfect for high security ; inbetween sendmail and qmail is smail, the best compromise. If you know what you're doing, choose sendmail (and you shouldn't be reading a HOWTO !); otherwise I generally recommand smail.