As others have pointed out, this is a very generic question and therefore difficult to answer accurately.

One idea might be to take a look at David Wood's book Programming Internet Email (O'Reilly). It gives a good overview of the various techniques for handling email and (unsuprisingly) most of the code examples are in Perl.

--
<http://www.dave.org.uk>

European Perl Conference - Sept 22/24 2000, ICA, London
<http://www.yapc.org/Europe/>