MESA alleviates this problem a bit by searching seven email directories simultaneously. If you have a name and you know which service provider the person uses , the best idea might be to go to the provider's homepage. They often have a searchable directory of their members. Note that this is not only true for commercial online services but also for many universities and companies. If that fails and you are looking for someone within a specific company or you can also try to send a mail to the postmaster.
Unfortuantely postmasters are permanentmy overworked, but it sure is worth a try.
Don't bother to try this with big online-services, like AOL, Compuserve etc. If you just have a name , it gets a bit complicated. In most engines you can do this by putting the name in quotes. This works best if the person you are looking for has a homepage or has published something online. Hower, it is a very poor method of searching if the person you are looking for has a very common name.