Este parece ser o " Modem de Pickman " ( ISFDB ) por Lawrence Watt-Evans. Foi de fato publicado em Asimov (embora em 1992, não nos anos 80).
Um resumo decente pode ser encontrado aqui :
As the story progress and the narrator, George, has brief Internet encounters with Pickman, the story spirals into the wonderful Mythos pitfalls that ensnare a large number of characters. Pickman and George get together and investigate strange messages. Pickman’s postings and emails are being rewritten after they are sent and before they reach the intended reader.