From Sender to Inbox: A Detailed Look at the Science of Email Delivery

Have you ever stopped to think about how emails are sent and received on the Internet? It might seem like magic, but there are actually some pretty straightforward protocols at work behind the scenes. In this article, we'll take a comprehensive look at two of these protocols: SMTP (Simple Mail Transfer Protocol) and POP3 (Post Office Protocol 3).

SMTP (Simple Mail Transfer Protocol) is a protocol for sending email messages between servers. Most email systems that send mail over the Internet use SMTP to send messages from one server to another, and to deliver messages to local mail clients like Microsoft Outlook or Apple Mail.

SMTP is a "push" protocol, which means that it is used to send messages from one server to another. When you send an email from your computer, your email client connects to your mail server and sends the message using SMTP. The message is then forwarded from your mail server to the recipient's mail server, based on the domain name in the recipient's email address (e.g. gmail.com).

Here's a simple example of how SMTP works:

1. Alice writes an email message and specifies Bob's email address (bob@gmail.com).

2. Alice's email client (e.g. Microsoft Outlook) sends the message to Alice's mail server, using SMTP.

3. Alice's mail server looks up the domain name (gmail.com) in Bob's email address and determines that the message should be sent to Google's mail servers.

4. Alice's mail server establishes a connection to one of Google's mail servers and sends the message.

5. Google's mail server receives the message and stores it in Bob's mailbox.

POP3 (Post Office Protocol 3) is a protocol used by email clients to retrieve email messages from a mail server. Most email clients (e.g. Microsoft Outlook, Apple Mail) use POP3 to retrieve messages from a mail server and store them on your computer. POP3 is a "pull" protocol, which means that it is used to retrieve messages from a server.

When you check your email, your email client connects to your mail server using POP3 and downloads any new messages from your mailbox. These messages are then stored on your computer, so you can read and respond to them offline. When you're ready to send a new message, your email client connects to the mail server again and sends the message using SMTP.

Here's a simple example of how POP3 works:

1. Bob checks his email using his email client (e.g. Apple Mail).

2. The email client connects to Google's mail server using POP3 and retrieves any new messages from Bob's mailbox.

3. The email client downloads the messages to Bob's computer and displays them to him.

4. When Bob is ready to send a new message, he writes it and specifies the recipient's email address (alice@yahoo.com).

5. The email client sends the message to Google's mail server using SMTP.

6. Google's mail server looks up the domain name (yahoo.com) in the recipient's email address and determines that the message should be sent to Yahoo's mail servers

7. Google's mail server establishes a connection to one of Yahoo's mail servers and sends the message.

8. Yahoo's mail server receives the message and stores it in Alice's mailbox.

It's worth noting that there are other protocols in addition to SMTP and POP3 that are used to send and receive email. For example, IMAP (Internet Message Access Protocol) is another protocol that is commonly used to retrieve email messages from a server. IMAP is similar to POP3, but it has some additional features that allow you to manage your messages more effectively, such as the ability to keep messages on the server even after you've downloaded them to your computer.

There are also other protocols that are used to send email, such as X.400 and MAPI (Messaging Application Programming Interface). These protocols are primarily used in corporate environments, and they offer advanced features such as support for attachments and encryption.

So that's a brief overview of how email works! SMTP is used to send messages from one server to another, and POP3 is used to retrieve messages from a server and store them on your computer. These protocols are an essential part of the modern Internet, and they make it possible for us to send and receive emails from anywhere in the world.

I hope this article has helped to clarify how email works and how SMTP and POP3 fit into the process. If you have any questions, don't hesitate to ask!