Your Internet Consultant - The FAQs of Life Online
The only exception that I know of is MCI Mail. Its users are listed in the Knowbot Information Service (see the answer to Question 4.20.)
If you have a CompuServe account, you can log in and check the user directory, but even that directory doesn't list all CompuServe users. Subscribers can elect to have their names and addresses taken out of the directory. Most other online services have similar directories that are only available to their own users.
Note: Some services will identify a subset of its users
if you try to send mail and it's not a unique descriptor. For example, there
are probably a half-dozen Dave Taylor's on CompuServe, so sending mail to
Dave.Taylor@compuserve.com might result in a message back from the
system indicating that there is more than one, and listing them. This doesn't
always work, but it's worth trying!