If you reply to a message received from the U.K., software at the gateways will automatically reorder the domains. However, if you are originating a message to the U.K., or if a correspondent has given you an address there, you must remember to reverse the order yourself, putting the host on the left and the .UK on the right, before sending a message. Unfortunately, there is nothing equivalent to the telephone directory, no file that lists all known e-mail accounts across the Internet. The surest way to learn a person's e-mail address is to ask that person. This may require using a tool other than e-mail, such as the telephone, surface mail, or a meeting in person.
Last names are the most common form of computer usernames.
However, there are some usernames that bear no relation to the person's actual name, so using finger becomes largely a matter of guesswork. As the White Pages project is still progressing, the information is currently incomplete, and not all sites are participating. As mentioned above, gateways are computers that sit on two or more networks, passing messages between the networks. They will frequently process an address received from one network into a form acceptable by another. In general, the best way to reach a user who is not on the Internet is to reply to a message sent to you.