Occasionally, if not frequently, one needs to search for the e-mail of an astronomer somewhere in the world. There are many ways to find out and several may have to be tried. You should start with the ``RGO email directory''. It is maintained by C.R. Benn and R. Martin and is made up of three parts. One is a list of ~ 13,000 personal emails, another one offers phone/FAX numbers and emails or URLs of ~ 950 astronomical research institutes, and a third one has postal addresses for ~ 650 institutions. You should make sure that your departmental secretary knows about the latter two! All lists are updated frequently and available for consultation at star-www.rl.ac.uk/astrolist/astrosearch.html. The impatient and frequent user of these directories should draw a local copy from time to time (three files at ftp.ast.cam.ac.uk/guide/astro*.lis), or ask your system manager to install a site-wide command to interrogate these lists and draw updated copies from time to time. To request inclusion in this directory or communicate updated addresses, send a message to email@ast.cam.ac.uk.
Be sure that you never (ab)use such lists of thousands of addresses to send your announcements to the entire list. You will most likely offend the majority of the recipients who are not interested in your message, which may be even regarded as ``spamming'' (see e.g. cdsweb.u-strasbg.fr/~heck/spams.htm for a collection of links to defend users from unsolicited email). However, these email directories may be useful for the legitimate task of selecting a well-defined subset of researchers as a distribution list for specific announcements.
The RGO email guide depends on personal and institutional input for its updates and turns out to be fairly incomplete especially for North American astronomers. Complete addresses for AAS members can be found in the AAS Membership Directory which appears in print annually and is distributed to AAS members only. The AAS membership directory has been put on-line experimentally and made searchable at directory.aas.org, but it cannot be downloaded entirely.
Since 1995 the IAU membership directory has been accessible at the LSW Heidelberg web site, but in early 1998 it moved to www.iau.org/members.html. The address database is managed by the IAU office in Paris (www.iau.org), and requests for updates have to be sent there by email (iau@iap.fr). From these the IAU office prepares an updated database every few months which is then put on-line. Hopefully this web address for the IAU membership directory will remain stable in the future, and not change every three years with the election of a new General Secretary at each IAU General Assembly.
The European Astronomical Society (EAS) is preparing its membership directory under URL www.iap.fr/eas/directory.html. It provides links to membership directories of several national astronomical societies in Europe (see www.iap.fr/eas/societies.html).
The ``Star*sFamily'' of directories is maintained at CDS ([Heck (1997)]) and divided into three parts. ``StarWorlds'' (cdsweb.u-strasbg.fr/~heck/sfworlds.htm) offers addresses and many practical details for ~ 6,000 organizations, institutions, associations, companies related to astronomy and space sciences from about 100 countries, including about 5,000 direct links to their homepages. ``StarHeads'' (cdsweb.u-strasbg.fr/~heck/sfheads.htm) is a compilation of links to personal WWW homepages of about 4500 astronomers and related scientists. For ``StarBits'' see Section 9.
Another way to search for email addresses is a search engine (mirrored at various sites) which can be accessed via telnet bruno.cs.colorado.edu, login as user netfind. You should give either first, last or login name of the person you look for, plus keywords containing the institution and/or the city, state, or country where the person works.
There are more, rather ``informal'' ways of tracing emails of astronomers. One is to check whether they have contributed preprints to the SISSA/LANL server (Section 6.2) recently. If so, the address from which they sent it will be listed in the search result returned to you. Another way is the command finger xx@node.domain, where xx is either the family name or a best guess of a login name of the individual you seek. However, some nodes prefer ``privacy'' and disable this command. One last resort is to send an email to postmaster at the node where you believe the person is or used to be.