1. A protocol packet or item of email (the latter is also called a letterbomb) that takes advantage of misfeatures or security holes on the target system to do untoward things.
2. Disapproving mail, esp. from a net.god, pursuant to a violation of netiquette or a complaint about failure to correct some mail- or news-transmission problem. Compare shitogram, mailbomb.
3. A status report from an unhappy, and probably picky, customer. “What'd Corporate say in today's nastygram?”
4. [deprecated] An error reply by mail from a daemon; in particular, a bounce message.