These types of mail bombs are simple to design but their extreme simplicity means they can be easily detected by spam filters.
This type of attack is more difficult to defend against than a simple mass-mailing bomb because of the multiple source addresses and the possibility of each zombie computer sending a different message or employing stealth techniques to defeat spam filters.
The attack can be carried out automatically with simple scripts: this is easy, almost impossible to trace back to the perpetrator, and potentially very destructive.
The attacker exploits web sites that allow Internet clients to register to some service with their Email address.
The list manager can therefore verify that the email in the form request matches the originating SMTP server in the validation message.
A new idea to combat this solution was composing a "bomb" consisting of an enormous text file, containing, for example, only the letter z repeating millions of times.
Such a file compresses into a relatively small archive, but its unpacking (especially by early versions of mail servers) would use a greater amount of processing, which could result in a Denial of Service.