mail bomb 
noun
1US A letter bomb.When we looked at that type of a scenario, whether it's anthrax letters or whether it's mail bombs, it is extremely difficult to investigate these cases....
- It is important to be alert for suspicious parcels, but keep in mind that a mail bomb is an extremely rare occurrence.
- Apparently our town doctor was the victim of a mail bomb.
2An overwhelming quantity of email messages sent to one address: what happens if advertisers don’t respond to entreaties and mail bombs?...
- In the fall of 1996, Synder was subjected to a mail bomb: an attempt to flood his electronic mailbox of someone with hundreds of email messages.
- Worm attacks, domain-name high-jacking, logic bombs, mail bombs and Trojan horses have all been utilized by hackers, but now denial-of-service attacks are gnawing away at the Internet's soft underbelly.
- Their ISP will cancel their account upon receipt of spam complaints or ensuing mail bombs.
verb
(mail-bomb) [with object] Send an overwhelming quantity of email messages to (someone): if the netizens are really annoyed they’ll mail-bomb you...
- The man in question has been mail-bombed a number of times and on one occasion received 250,000 emails within just a couple of hours.
- EFF Action Alerts are issued regularly on subjects of interests to the organisation, and are intended to help coordinate activism and/or to mail-bomb unlovable companies, agencies and legislators.
- Posting "spam", and/or posting binaries to non-binary newsgroups, can result in people being mail bombed.