The Jargon File (version 4.4.7, 29 Dec 2003):

nastygram
 /nas'tee?gram/, n.

    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.


The Free On-line Dictionary of Computing (30 December 2018):

nastygram

    /nas'tee-gram/ 1. A network packet or
   e-mail message (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 e-mail, especially 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 did Corporate say in today's nastygram?"

   4. [deprecated] An error reply by mail from a daemon; in
   particular, a bounce message.

   [Jargon File]

   (2004-02-17)