Once we know how an e-mail looks, the next step is see how it’s delivered from end to end. When the e-mail has written in the mail application (for example, Outlook of you are using Windows or simply Mail if you are using Mac OS X) it must be send to the destination, but this is usually a multistep process.

In first place, the mail application must use the SMTP (Simple Mail Transport Protocol) protocol to send the message to the server we are using. This server is assigned by our ISP and usually it’s name is smtp.ourisp.com or similar to this. We can see an example of the interchange of data in a tipical SMTP session (> represents data send by the server and < data send to the server):

> 220 smtp.ourisp.com SMTP Sendmail
< HELO 192.168.0.1
> 192.168.0.1 Hello, pleased to meet you
< MAIL FROM:
> 250 ... Sender ok
< RCPT TO:
> 250 ... Recipient ok
< DATA
> 354 Enter mail, end with "." on a line by itself
< [...head and body of e-mail...]
< .
> 250 LAA06495 Message accepted for delivery
< QUIT
> 221 smtp.ourisp.com closing connection

Once this has concluded, the SMTP server will try to send the e-mail to the destination server, perhaps using other intermediate servers, using the same protocol until it gets to the destination where the recipient will be able to download it.

The process of downloading mail can be done with two different protocols: POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol). Historically, there have been other protocols, ancient versions of these, but these are nowadays the defacto standards. There are some differences between them and we can say that IMAP is more advanced than POP3, but there many servers which don’t use IMAP yet, so it’s not as widely extended.

We are not going to explain the details of these two protocols, suffice to say that they are used to get the mail to our computer. If you need more technical details you can consult the book Programming Internet Email.

Once the e-mail has arrived to our computer, it’s our decision what to do with it, whther we are interested in it or we are going simply to delete it. But when the quantity of undesired mail is really high, the process might be long and painful. We’ll see techniques to deal with this in later parts of these series.