Several programs exist to query a phonebook server (CCSO). In the access list you can find out about them and how to configure them.
Phonebooks are also accessible through WWW, with or without special forms.
Usage of those forms is quite straightforward.
For those sites who do not offer a form to query a phonebook server, this small introduction on querying a phonebook is available.
The FAQ find-email-address contains a list of methods and ways to find email addresses at several places.