What is POP3 and SMTP?


Short for Post Office Protocol version 3, POP3 is the most common protocol for receiving email messages. An email server stores messages that have been sent to you until you use your email client to retrieve them using POP3. See also IMAP and SMTP.


Short for Simple Mail Transfer Protocol, SMTP is the protocol most commonly used to send email messages. Often messages are passed among several SMTP relay servers before reaching their final destination.

Glossary of Telecom Terms


Asymmetric Digital Subscriber Line. A digital line that uses the existing twisted pair copper telephone network to achieve speeds of up to 6 megabits per second up to 12000 feet, or 1.5 megabits per second up to 18000 feet.


Providing differing bandwidth in different directions. 56 K modems are asymmetrical: they offer a maximum speed of 56K for downloading, but only 28.8K or 33.6K for uploading.


A measure of signal changes per second. Often used incorrectly in place of bps (bits per second).


bits per second


Basic Rate Interface. A consumer grade ISDN line consisting of 2 64K bearer channels and one 16K delta (controller) channel.


Competitive Local Exchange Carrier. An alternative to the existing local phone company.


Central Office, in reference to the phone company’s central switching station for a given area.


Converting analog signals back into digital signals. A modem is a MOdulator/DEModulator.


Domain Name Service (DNS) is used to determine the physical Internet address (IP address) for a domain’s mail host so that a server or gateway can connect and send it mail. DNS is a service that maintains a database of host names, IP addresses, and domains. DNS also stores information like domain name aliases and routing records (called MX records). If a DNS server is available (usually at your Internet Server Provider’s site), the gateway can request from it a domain’s mail host and network address.


A high-speed line capable of delivering 1.54 Mbps (1,540K) in both directions, and divided into 24 data-bearing channels.


A high-speed line capable of delivering 3.15 Mbps (3,150K) in both directions.


A high-speed line capable of delivering 6.31 Mbps (6,310K) in both directions.


A high-speed line capable of delivering 44.7 Mbps (44,700K) in both directions.


Roughly the European equivalent of a T1 or a PRI, but with 30 data-bearing channels


A device that converts the two-wire local loop to the four-wire central office.


Integrated Services Digital Network. A digital phone service capable of speeds from 57.6 K to 128 K. Provides two data channels, each with its own phone number, making simultaneous voice and data possible.


BayouCity.Net – Internet Service Provider. A company that provides access to the Internet through modems, ISDN, T1s, etc.


Lucent and Rockwell’s joint 56K modulation protocol that makes Rockwell’s K56Plus and Lucent’s V.flex2 technologies interoperable


Rockwell’s proprietary protocol for 56000 bps modulation. Merged with Lucent’s V.flex to create K56flex.


kilobits per second. KBps is kilobytes per second.

local loop

The copper wires running between the telephone subscriber’s home or business and the phone company switch.


megabits per second. MBps would be megabytes per second.


A MOdulator/DEModulator. A device that can encode digital signals from a computer into analog signals that can be transmitted over analog lines, and vice versa.


Converting digital signals into analog signals. A modem is a MOdulator/DEModulator.


A fiber optic line capable of 155 megabits per second (155,000K).


A fiber optic line capable of 2400 megabits per second (2,400,000K).


Pulse Code Modulation. A method of encoding an audio signal in digital format.


Personal Computer Memory Card International Association. A standard for miniaturized laptop expansion cards for modems, storage, and other devices. Often called PC cards.


Point of Presence. A local dialin point for an Internet Service Provider.


Plain Old Telephone Service. Regular analog phone service, as opposed to ISDN, ADSL, and other digital phone services.


POP3 is an electronic mail protocol used to retrieve messages stored on an Internet/intranet e-mail server.

POP3 is a ‘pull’ protocol. Whenever a client wants to check for messages it connects to its Internet Service Provider’s e-mail server and uses POP3 to login to its mailbox and ‘pull’ down its messages.

POP3 is well suited for dial-up environments because the client need not be attached to the Internet/intranet when another user is trying to send it mail. The mail arrives at the server and is stored in the user’s mailbox. The server is always available on the Internet/intranet to receive mail. The client can dial-up and attach to the server at a later time to retrieve its messages.

POP3 Server

A POP3 server receives and stores e-mail messages on behalf of its users. Its users can connect to the server using Internet/intranet clients (that support the POP3 protocol) and retrieve their messages. Users may also send messages via the server (using SMTP).


Primary Rate Interface. An industrial grade ISDN line. In the United States and Japan, a PRI consists of 23 64K bearer channels and a 64K delta (controller) channel. In Europe, a PRI consists of 30 bearer channels and a delta channel.


Public Switched Telephone Network.


Regional Bell Operating Company.


In North America, a digital carrier for a DS1-formatted signal.


SMTP is an electronic mail protocol used by Internet/intranet based email servers and gateways to exchange messages.

SMTP is a ‘push’ protocol. Whenever an SMTP server or gateway has a message to send, it establishes an SMTP session with the destination server or gateway and then transmits (‘pushes’) one or more messages. The receiving server or gateway must be ‘listening’ for an SMTP connection request. If the destination SMTP server or gateway is not attached to the Internet/intranet, message exchange is not possible. The transmitting server or gateway will repeatedly try to connect to the destination server or gateway. After a certain number of failed attempts the message will be discarded.

SMTP is well suited for servers or gateways that are continuously attached (using a dedicated connection) to the Internet/intranet. Servers or gateways that are not continuously attached (using a dedicated dial-up connection) to the Internet/intranet may use SMTP but they must attach to the Internet/intranet for sufficient periods of time (usually hours). This is to ensure that they are available to any server or gateway trying or retrying to send it messages. Dedicated dial-up SMTP, therefore, is not efficient because a connection needs to be maintained even when messages are not being transferred. You pay for the connection time and need a telephone line that cannot be used for any other purpose (i.e. shared with fax machine or individuals surfing the Internet). Some SMTP servers support ways of triggering them to start sending mail to a specific domain. If your ISP supports this feature then the dial-up connection need only be maintained while messages are being exchanged.


In North America, a digital carrier for a DS3-formatted signal.


The ITU standard for 56K modulation.


U.S. Robotics’ proprietary protocol for 56K modulation.
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