Electronic mail programs do not provide you with the equivalent to a telephone directory: there is no way that you can look up a person's electronic mail address.
Many people now include their email address on their business stationery. And of course you can always ask them for it.
The "header" of any mail that you receive will contain the email address of the person who sent it, in the From: field. For example:
Date: Tue, 18 Aug 92 12:15:17 WET DST From: tony@com/cayman Subject: Xgator troubles To: email@example.com
Each domain has a "postmaster" who will try to supply you with a mail address if asked. For example:
From: firstname.lastname@example.org Subject: Address please To: postmaster@durham Please provide the email address for Dr J.R. Smith who is a member of staff in the Psychology Dept. Thankyou