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| 7.3 |  | Troubleshooting TCP/IP Application Layer Protocols |
| 7.3.5 |  | Electronic mail |

Simple Mail Transport Protocol (SMTP) is used to transport e-mail messages in ASCII format using TCP between clients and servers. Other protocols such as Post Office Protocol (POP) or Internet Access Message Protocol (IMAP) are used to retrieve e-mails from mail servers.

For example, assume User-A wants to send e-mail to User-B. When User-A clicks on the Send button, e-mail is sent to the local e-mail server using the SMTP protocol. The e-mail server will then send the e-mail using SMTP to User-B’s e-mail server. It remains stored there until User-B collects it. Later, User-B connects to the local e-mail server and downloads the e-mails using either POP3 or IMAP 4.

POP v3 is the current version of the protocol and it is incompatible with earlier versions. POP3 downloads user e-mails to the local computer. For this reason, POP3 is best suited in situations where users retrieve their e-mail from the same computer. If users use different computers, their e-mails will likely be spread around several computers.

IMAP v4 is another alternative that lets users download their e-mail at any time to any computer.

Because these different protocols are used to send and receive mail, it is possible that mail clients can perform one task and not the other. Therefore, when verifying the configuration of a mail client, both the mail relay (SMTP) server and mail (POP or IMAP) servers should be verified.

SMTP, IMAP, and POP connectivity can be tested using any Telnet application that allows a port number to be specified. Telnet to the IP address of the destination server using ports 25, 143, and 110 respectively.

The following commands can be used to isolate application layer problems related to email and the POP3, SMTP, and IMAP protocols.

If there is a problem with the receiving system, the user should see a text error message. If the connection was successful, a hello message will be displayed or an unresponsive Telnet window will open. This indicates connectivity to the server. At this point, the user could use POP3 or SMTP text-based commands to perform basic e-mail procedures such as authenticate, read, delete, or send messages. For example, basic POP commands include user, pass, stat, list, top, uidl, retr, dele, noop, rset, and quit.