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.