The title is something of a non sequitur in that nowhere in the novel does a postman character appear, nor is one even alluded to. As such, its meaning has often been the subject of speculation by writers such as William Marling, who suggested that Cain may have taken the title from the sensational 1927 case of Ruth Snyder. Snyder was a woman who, like Cora in The Postman Always Rings Twice, had conspired with her lover to murder her husband. In that case, Snyder said she'd prevented her husband from discovering the changes she'd made to his life insurance policy by telling the postman to deliver the policy's payment notices only to her, and instructing him to ring the doorbell twice as a signal indicating he had such a delivery for her.
In fact, however, Cain gave a specific explanation of the title's origin in the preface to his subsequent novel Double Indemnity. There, he wrote that the genesis of the title for The Postman Always Rings Twice came from a discussion he'd had with screenwriter Vincent Lawrence. According to Cain, Lawrence spoke of the anxiety he felt when waiting for the postman to bring him news on a submitted manuscript. According to Cain, Lawrence noted that he would know when the postman had finally arrived because the postman always rang twice, and Cain then lit upon that phrase as a title for his novel. Upon discussing it further, the two men agreed such a phrase was metaphorically suited to Frank's situation at the end of the novel.
With the "postman" being God, or Fate, the "delivery" meant for Frank was his own death as just retribution for murdering Nick. Frank had missed the first "ring" when he initially got away with that killing. However, the postman rang again, and this time the ring was heard, when Frank was wrongly convicted of having murdered Cora, and then sentenced to die for the crime. The theme of an inescapable fate is further underscored in the novel by The Greek's escape from death in the lovers' first murder attempt, only to be done in by their second one.
In his biography of Cain, author Roy Hoopes also recounted the conversation between Cain and Lawrence that gave birth the novel's title. Hoopes's account of their conversation is similar to Cain's, but offered extended detail regarding Lawrence's comments. Specifically, in Hoopes's telling, Lawrence did not say simply that the postman always rang twice, but rather said that at times he was so anxious awaiting the postman's delivery that he'd intentionally go into his backyard trying to avoid hearing the postman's ring. However, Lawrence continued, this tactic inevitably failed because even if the postman's first ring was not noticed, he would always ring twice, and that second ring would inevitably be heard.
In the 1946 film adaptation of the novel, Frank explicitly explains the title in the terms offered by Hoopes's biography of Cain.