The problem of backscatter, part 2 - The legitimate case

Before getting into the problem of backscatter, let's look at how the system
is supposed to work before spammers ruined it for everyone.

Let's say that you want to mail a letter to your friend.  You write the
letter, put it in an envelope, and write your friend's address in the center of
the front of the envelope.  You then put your address on the top left corner of
the envelope, put a stamp on it and then walk down to the nearest mailbox and
drop it in the slot.  The post office comes, picks up the letter and then
through some process known as magic, a few days later your friend gets your

However, suppose there's a problem.  Let's say you write the letter to your
friend and address it this way:

Homer Simpson
771 Evergreen Terrace
Springfield, USA

Aside from the fact that Homer lives at 742 Evergreen Terrace (or 743
depending on the episode), you have not specified either the state or the zip
code where Homer lives.  The post office sees this and is unable to deliver your
mail so they mark it and return it to you since you put your return address at
the top of the envelope.  On the letter, they put notices like "Bad address" or
"Insufficient Postage" or something similar.  In other words, they mark the
message as non-deliverable.

Email works the same way.  You write an email, put your name and email
address in the P1 From (SMTP MAIL FROM) and address it to your friend, who you
put in the P2 From (SMTP RCPT TO).  You hit send in your email client and by a
process known as magic, your email eventually gets delivered to your friend in a
matter of seconds.

But what happens if you put a typo in your email address?  Just like the post
office, the email postmaster has ways of letting you know that your message did
not go through.  Suppose you did this:

From: Homer Simpson <hjsimpson @>
To: Krusty
the Klown <krustyClown @>

But, Krusty's email address is actually krustyKlown @  Krusty's
recipient mail server gets Homer's email, looks at the To: address and then
tries to deliver the mail.  But oops!  It sees that the email address doesn't
exist so it sends a notification back to Homer that the message could not be
delivered because the email address that he specified was invalid.  This is
known as a Non-Deliverable Receipt (NDR) or a Delivery Status Notification
(DSN).  Suffice to say, the email postmaster Homer has been sending to has been
kind enough to notify you that your message did not go through.  You get the NDR
back in your own email inbox so you can take action on it.

In my next post, I'll go into a bit more detail about how this process works
in the legitimate case.