Email Explained


Email Explained

How does email work? What’s the difference between POP and IMAP? Why do you need a port number to connect to a mail server? Why do people pretend to be Nigerian princes needing money? Do I need my own mail server or my own email client? This guide will help you better understand these and other issues surrounding email.
How Email Works

An email begins as a message composed by the sender using a Mail User Agent (MUA.) When the user sends the email, the Mail Submission Agent (MSA) formats the email according to the Simple Mail Transfer Protocol (SMTP) standard, then the message is transferred to the sender’s mail server using a Mail Transfer Agent (MTA.)

The server uses its own MTA to find the IP address for the receiver email’s domain name, the site after the “@” in their email address. The email may need to be sent through several servers before it arrives at its destination. Each time, an MSA adds location information and an MTA sends the message to the next server. Once the message reaches the receiver’s server, a Mail Retrieval Agent (MRA) transfers the message to his or her MUA where it can be viewed.

Although all the software can be separate, it’s usually combined into single programs with an email client handling all the steps for the end users and server software performing MSA and MTA tasks. Here’s an example of how a typical email is sent:

– The sender writes an email from to
– The email client transfers the message to the server.
– The server looks up IP addresses for the domain name “” and finds a route to’s mail server.
– The email is transferred to through’s server adds its identity to the email header before sending the message to
– The receiver’s email client downloads the message from

Email Transfer Protocols

When sending an email, servers use Simple Mail Transfer Protocol (SMTP,) while there are several protocols used to receive emails. Most emails are received in one of four ways:

POP – Post Office Protocol is a simple system that sends all emails from the server to the MUA when accessed. Once the messages have been received, they’re deleted from the server.

IMAP – Internet Message Access Protocol offers more flexibility than POP, adding features like giving users the option to keep messages on the server after they’ve been received and IDLE mode which lets the user turn active message notifications from the server on and off to save bandwidth. These features make it a better choice over POP when an account needs to be accessed from several devices.

MAPI – Messaging Application Programming Interface, also called Microsoft Exchange or Exchange ActiveSync, is Microsoft’s mail transfer protocol for Exchange. Functionally, it’s similar to IMAP.

Webmail – Services like Google Mail and Yahoo! Mail combine the mail server with an online client, letting the user access their mail using a web browser. These services usually offer the option of POP, IMAP or MAPI access to allow users to use a separate client with the account.

Ports and Mail Server Access

TCP/IP standards specify 65,535 ports for each IP address with each port acting as a channel for a specific type of data. Some services use standard ports: the page you’re reading now was probably transferred through port 80. However, there’s no standard port for mail services. Most SMTP transfers are done on port 25, 2525 or 587, but the administrator of the server could be set transfers to any port they want to. This is why both server addresses and port numbers are needed to connect with mail servers.

Of course, ports can also be used to transfer malicious data like viruses, which is why firewalls are recommended on Internet-connected devices. A firewall blocks data transfer from ports that aren’t commonly used, but that may mean blocking a port needed to connect with the server. To get around this, the firewall must be told to “open” the port.

Email Clients vs. Webmail

At one time, email clients offered major advantages over web-based email access. They can be used off-line, sending and receiving messages when reconnected to the Internet. They can handle several email accounts, automatically separate messages into different folders and can apply custom filters to remove spam. However, with the advent of cloud computing and web application programming languages like HTML5, the line between the two categories has blurred as web interfaces have added features that were once available only in email clients.

Today, choosing between the two is a matter of picking the features available from an individual platform. For example, Microsoft has turned their Hotmail service into Both stand-alone Outlook and work similarly, but Outlook works better with Office while has added features for the web-based Office365.

What is Spam?

Spam is the online equivalent of junk mail, not only in how it looks but how it functions. Spammers buy or build lists of emails which they send email to. However, while mailing a flier may cost an advertiser a dollar or more, it costs no more than a penny when hiring professionals to send emails. This makes it practical to advertise scams like illegal pharmaceuticals or “419” requests for money from Nigerian princes. If only one person in thousands falls for it, it’s still cost effective. Spam can also take the form of “worms,” viruses that access email accounts and mail themselves to anyone listed in the user’s address book or sent mail folder.

This has led to a race between software developers and spammers trying to control what ends up in users’ inboxes. Strategies used to block spam include checking for shortened links, corrupted attachments, a lack of text and unusually poor syntax. IP addresses that have previously sent span will also be flagged. In either case, it’s possible for non-spam messages to accidentally be flagged.

Email Service vs. Private Servers

The choice comes down to one of convenience versus privacy and security. Setting up a mail server means having to do all the system administration that comes with an email service, including security and spam filtering. However, if the server is on-site, emails sent between local users will never leave the site unless system security has been compromised. Owning a server also means users don’t have to agree to a provider’s terms of service, which can include advertising and the use of emails for gathering user data. Having a separate domain name also means it is less likely that emails will be flagged as spam since the IP address will only be sending out legitimate email.


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