What Is Spam

The definition of spam is an issue of opinion. The US government has essentially defined spam as email that fails to comply with the CanSpam Act. A number of states have enacted their own laws. Each of us is free to use whatever criteria that we feel is appropriate. My notion of what constitutes spam follows:

Unsolicited Bulk Email (“UBE”)

Unsolicited: We didn’t subscribe to it. Bulk: Commercial and non-commercial email, apparently destined to multiple recipients. Most unsolicited commercial email (“UCE”) falls into this definition. CanSpam compliance is irrelevant.

Repeatedly Rejected Mail

This generally applies to solicited bulk email. Once a user moves on and his or her email is rejected, it is reasonable to expect that bulk emailers review their logs and manage their lists. It doesn’t take long for the average company, with moderate employee turnover, to start rejecting more email for former employees than they accept for current employees. While each reject consumes a trivial amount of cycles and bandwidth, over time this can become a voracious consumer of resources.

Challenge Response (“C/R”) Challenges

C/R is a spam filtering system. When email is received from an unknown sender a “challenge” is sent back to the sender asking them to confirm, by return email or web page, that the email is valid. The theory is that spammers won’t respond.

This issue can stir up some very acrimonious debate. Sorry but C/R is a terrible idea. Spammers generally use the email addresses (often valid) of others as the “sender.” Therefore, the majority of challenges will be sent to people who are unrelated to the email being challenged. As far as we are concerned, this is spam.


Mail servers should be configured to provide non-delivery notices (bounces) only to local users. Unaccepted email from outsiders should be rejected. This prevents the purported sender from receiving bounce messages for mail that he or she never sent. As far as we are concerned, backscatter is spam.

Here is an example:

Spammer does a spam run from a server in South Korea. While the real sender is nitwit@BadGuys.kr, the purported sender is our friend exmachina@tqmcube.com. One of the intended recipients is NoSuchUser@Company.TLD

If company.tld has a properly configured mail server, the SMTP conversation looks like this:

HELO forged.domain.name
MAIL FROM: exmachina@tqmcube.com
RCPT TO: NoSuchUser@Company.TLD
550 5.1.1 unknown user: NoSuchUser — 550 is a reject

In this case, the outgoing server at BadGuys.kr creates the bounce message which is sent to its local user “Nitwit.”

Backscatter is created if the mail server at company.tld accepts the message and then is unable to deliver it to local user “NoSuchUser”. Then the server returns the message to the forged sender, exmachina@tqmcube.com

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