|  | 15.3 | Network Applications
| 15.3.3 | DNS function
Whenever e-mail clients send letters, they request that a DNS connected to the network translate the domain names into their associated IP addresses. If the DNS is able to translate the names, it returns the IP addresses to the clients, thus enabling proper transport layer segmentation and encapsulation. If the DNS cannot translate the names, the requests are passed on until the names can be translated. The part of the e-mail address that contains the recipient's name becomes important at this point. The server extracts it from the e-mail message and checks to see if the he/she is a member of its post office. If the recipient is a member, it stores the message in his/her mailbox until someone retrieves it. If the recipient is not a member, the post office generates an error message and sends the e-mail back to the sender. The second part of the e-mailing process is the receiving process. E-mail message recipients must use the e-mail client software on their computers to establish requests to the e-mail post offices. When message recipients click the "Get Mail" or "Retrieve Mail" buttons on the e-mail client, they are usually prompted for a password. After they enter the password and click "OK", the e-mail software builds a request for the post office servers, which then extract the post office addresses from the configuration data that was entered when their e-mail software was configured. The process then uses another DNS search to find the IP addresses of the servers. Finally, the requests are segmented and sequenced by the transport layer. Data packets travel through the rest of the OSI model layers (i.e. network, data link, physical) and are then transmitted across the Internet to the destination e-mail post office. At this post office the packets are reassembled, in the proper sequence, and are checked for any data transmission errors. At the post office, requests are examined, and user names and passwords are verified. If everything is correct, the post office server transmits all e-mail messages to computers, where the messages are, again, segmented, sequenced, and encapsulated as data frames, to be sent to the client's or the e-mail recipient's computer. After e-mail messages arrive at a computer, you may open them and read them. If you click on the "Reply", or the "Forward" button, to send a response to a messages, the whole process starts over again. E-mail messages, themselves, are normally sent as ASCII text, but the attachments that are sometimes included with them, can be audio, video, graphic, or many other types of data. To correctly send and receive attachments, the encoding schemes must be the same on both the sending and the receiving computer. The two most common formats for e-mail attachments are the Multipurpose Internet Mail Extension (MIME) and UUencode (a Unix utility program).