what is email bomb?

An e-mail bomb is a form of net abuse consisting of sending huge volumes of e-mail to an address in an attempt to overflow the mailbox or overwhelm the server where the email address is hosted in a denial-of-service attack
 
Methods of email bombing
There are two methods of perpetrating an e-mail bomb: mass mailing and list linking.

 
Mass mailing

Mass mailing consists of sending numerous duplicate mails to the same email address. These types of mail bombs are simple to design but their extreme simplicity means they can be easily detected by spam filters. the attacker wishes to flood. This form of email bombing is similar in purpose to other DDoS flooding attacks. As the targets are frequently the dedicated hosts handling website and e-mail accounts of a business, this type of attack can be just as devastating to both services of the host.

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.
 
List linking

List linking means signing a particular email address up to several email list subscriptions. The victim then has to unsubscribe from these unwanted services manually. In order to prevent this type of bombing, most email subscription services send a confirmation email to a person's inbox when that email is used to register for a subscription. This method of prevention is easily circumvented: if the perpetrator registers a new email account and sets it to automatically forward all mail to the victim, he or she can reply to the confirmation emails, and the list linking can proceed.

Zip bombing

A ZIP bomb is a variant of mail-bombing. After most commercial mail servers began checking mail with anti-virus software and filtering certain malicious file types, EXE, RAR, Zip, 7-Zip. Mail server software was then configured to unpack archives and check their contents as well. A new idea to combat this solution was composing a "bomb" consisting of an enormous text files, 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...