There's no big issue. Main steps:
- set up an MX record for each of the customer domains pointing to your mail server;
- configure Sendmail on the server to treat these domains as local and to properly rewrite addresses;
- run a POP server on your machine;
For furhter info:
http://sendmail.net/smfaq_virthost.shtml
(note that from v8.10 sendmail.cw is called
local-host-names)
-- dolom --