dowco.com News: Email Primer
Edwin Parcero

E-mail is a way of sending and receiving messages over an electronic system.  It passes from one computer to another (aka, mail servers), just like a letter makes its way at different postal stations to reach its final destination.  Once it arrives at the destination mail server, it's stored there until the recipient retrieves it.  A big advantage over snail mail is that the whole process can take a few seconds.  This allows you to quickly communicate with anyone at anytime.

To send and receive an e-mail, you need a few things:-

• a network/internet connection
• a program that knows how to communicate with the mail server (eg. Outlook Express, Thunderbird, any web-based client program, etc.) that is used to pickup and send an e-mail. Other email clients have more features, but at the core, they all work the same way :-)
• an account on the mail server, either with your ISP or on your own mail server.

The server is an application that runs on a machine and listens to specific ports waiting for other programs (eg, mail clients) to connect to it.  Port 110 is normally used by the POP3 server, port 25 is the one used by the SMTP server, while IMAP uses port 143.  To avoid conflicts, each type of server uses different ports.

When you pick up your email on a POP3 server, your client program will connect to port 110.  It passes on the username and password and once authenticated, emails should start coming through.

When you send out an email, your client program will try to connect to port 25 of your SMTP server.  Once the server receives the e-mail, it breaks it into two parts, the header and the data.  The data is the part you normally see when you send/receive an e-mail, while the header gets updated by the mail server when it receives the email.  When you check the header, you can trace where the email goes to before it arrives to your ISP's mailbox.

Here's an example of what an email header looks like.  As you can see, the mail servers will add some info (name of machine and time) on it once it receives the e-mail.

Received: from neptunium.dowco.com (localhost [127.0.0.1])
by neptunium.dowco.com (8.12.2/8.12.2) with ESMTP id h1DJ6kbk018897
for < This e-mail address is being protected from spambots. You need JavaScript enabled to view it .PROCMAIL>; Thu, 13 Feb 2003
11:06:46 -0800 (PST) (envelope-from This e-mail address is being protected from spambots. You need JavaScript enabled to view it )

An e-mail message is nothing more than a simple text message, and that's even if you send out a non-text attachment (eg, pictures, pdf files, etc). When the server receives an attachment, it converts it to a text format that looks something like this.

---MOQ104516320685bd7380598
Content-Type: image/pjpeg; name="bkgvoltezv.jpg"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="bkgvoltezv.jpg"

/9j/4AAQSkZJRgABAQEASwBLAAD/2wBDABALDA4MChAODQ4
ODdASFxOQERXRTc4UG1RV19iZ2hnPk1xeXBkeFxlZ2P/2wBDARE
Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY
AhEBAxEB/8QAHwAAAQUBAQEBAQEAAAAAAAAAAAECAwQF

You don't have to worry about this, since both the server and the client program will convert the attachments automatically.  The size of the attachment will be different once it gets encoded on the server.  Mail servers normally have a limit on the size of the e-mail attachment that it's allowed to send out.  So, if your ISP's mail server has a 5 Mb limit, don't be surprised if your 4.5 Mb e-mail gets rejected.

Other things that could cause problems with sending/receiving e-mails are anti-virus programs and firewalls.  Make sure they are configured and working properly.

Though there are parts of the e-mail system that do get complicated (eg. routing rules in sendmail, encoding/decoding of attachments, mail filtering, etc.), the basic system is quite straightforward.