Finding e-mail addresses
Directory services pages:
- Ph directory services page, especially if you know a lot of students, faculty, and staff at colleges and universities (although Ph, like LDAP, can be used by any organization).
- LDAP directory services page.
- Whois lets you look up who owns a domain as well as their contact information.
- Finger! What is "finger" and how to use it.
Change of address directories/services:
- FreshAddress.com, the Free Change of Email Address Directory. "The Internet's largest directory of old and current email addresses, which provides the missing link between the two. Free searches by name or even OLD email address. If the email address you're searching for isn't registered, the site will keep searching and inform you when it is."
- Re-Route. E-mail messages sent to your old AOL or ISP address are retrieved by the Re-route server, re-addressed and sent on to your new e-mail address. "Change of address" notices are automatically delivered to the original senders informing them whenever their messages have been "re-routed."
- ReturnPath
The best method is to just ask someone; if they don't know what their e-mail address is, give them yours and ask them to send you a message, keeping in mind that if someone doesn't know their e-mail address, they probably don't use e-mail that much.
Links:

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