One of the most experienced and clever editors I know has suffered from email abuse from a so-called literary agent.
No reputable agent would ever harangue an editor after a turn-down, however disappointed they were on behalf of their much-loved author. The editor now suspects that this IS an author posing as an agent and sounding very threatening.
This happened to me - got successively abusive (hand-written) letters followed by death-threats for turning down a novel from someone who claimed to be in the SAS. In the letters he talked about his SAS experience and 'taking us all out' so we called the police and they went round and spoke firmly. Never heard a word again...