Email is one of the Internet's oldest applications, and it's the most widely used. Internet users send hundreds of millions of email messages to one another every day — and the majority of messages are business-related.

Considering how important email has become in our business communications, it's surprising how few people understand how email actually works. While the process is complex on many levels, understanding how email travels from point A to point B is actually quite simple.

Sending Mail
When you send an email message, your email program connects to your Internet service provider's mail server. This is typically a Simple Mail Transfer Protocol (SMTP) server, the type of server responsible for sending email.

Your message might be broken into smaller pieces, or packets, before it begins its journey. These smaller packets can travel more quickly from server to server and are reassembled when they reach their destination.

Sorting the Mail
In the same way that postal mail is sorted first by the post code, email messages are sorted by domain. For the email address feedback@allbusiness.com, for example, the domain is the "allbusiness" portion. The domain identifies where the message needs to go.

Each domain name maps to a unique Web address, called an Internet protocol (IP) address, which is a string of numbers that servers use to route messages. These relationships are stored in the Domain Name Registry. When the SMTP server receives a message, it looks at the domain and checks the registry to determine what IP address to send the message to. Once it determines the proper server, the SMTP server sends the email message on its way.

Delivering the Mail
Depending on where the destination server is, the original SMTP server may not actually make the final handoff; it may pass the message to another server. That server identifies the domain and passes the message to another server. This process is repeated, the message getting closer and closer to its destination, until the correct server is reached.

The server that receives mail is generally a post office protocol (POP) server. ISPs set up user mailboxes on POP servers. Usernames are like post office box numbers, and passwords act as keys that open the correct box. Once a message reaches the appropriate domain server, it is channeled into the right POP account and stored until the user logs in and checks for mail. In other words, this is how the feedback@allbusiness.com address is differentiated from the service@allbusiness.com address.

When the recipient tells their email program to check for new mail, the email program connects to the ISP's POP server, looks in the user's mailbox and retrieves any mail that's waiting. The message's journey is complete — and all this might have happened in a matter of seconds.