("Guilty" may be the wrong word, because it's not a criminal offense, but I'm not sure what the right one is here.)
Defamation is a crime in many jurisdictions, including the US (not federally, but in over 20 states). Prosecutions and convictions are rare-but not non-existent. At least in the US, punishments are light (Wikipedia cites a study saying that the average custodial sentence for criminal libel in the US is just under six months). In free countries, prosecutions are generally reserved for particularly egregious cases (e.g. suppose A does some trivial thing to offend B, so as payback B plasters posters all over town accusing A of sex crimes, with the allegation being completely invented). Given this, the vast majority of defamation cases are civil not criminal.