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IT Essentials — Assessing Infrastructure and Networks

Messaging Protocols

Several protocols (message transfer rules) have been developed to administer and govern the
transfer of messages among computers on a network. There are several message-related protocols
that govern how messages are sent, received, and queued. An easy way to think of a protocol is to
consider it similar to a language. For two devices to communicate, they must establish rules of the
language they will follow.

Transmission Control Protocol/Internet Protocol (TCP/IP) defines the rules for how data is sent and
received over a network. TCP/IP is the baseline protocol that supports internet communication and
all other protocols leverage TCP/IP.

Simple Mail Transfer Protocol (SMTP) governs how email messages are sent and received. Messages
must be queued because users are not necessarily immediately available to consume them.

Messages are consumed using one of two queuing protocols, Post Office Protocol (POP) and Internet
Message Access Protocol (IMAP):

POP messages are received and stored on an email server. When these messages are consumed,
they are downloaded to the consumer’s device. Messages are not retained on the server once
consumed.

IMAP messages are received and retained on an email server. When these messages are consumed,
they can be organized into various folders rather than being downloaded to the consumer’s device.
Messages are retained on the server once consumed, thus IMAP can be thought of as a file server for
messages.

Email Delivery Process

Most organizations have a unique email domain (the content that comes after the @ symbol in an
email address), which is considered a local domain. This local domain is managed through a mail
server, also known as a mail (or message) transfer agent (MTA). This server can be administered by
the organization or through a third party or cloud service.

Email is composed and delivered using an email client, which is either a web-based application, such
as Gmail, or by using a dedicated application on a user’s computer, such as Microsoft Outlook. The
email client is also called the Mail User Agent (MUA).

When a user sends an email, it transmits to the mail transfer agent (MTA), which collates and
distributes internal email (messages within the same domain). It also distributes outgoing email to
external users (outside the domain).

Each mail user (MU) is assigned a unique email address, with the format of user@domain.com. This
corresponds to a “mailbox” to which the MTA will deliver all incoming messages. The MTA will also
label all outgoing mail from the mailbox with the user’s unique email address.

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