Poison Pen

The calm of a peaceful English village is shattered when a series of anonymous letters starts being delivered to village homes, containing scurrilous allegations about the recipients and their families. Upstanding and respectable inhabitants find themselves and their loved ones accused in lascivious detail of all manner of moral, sexual and criminal misdeeds. The Reverend Rider (Tate) and his sister Mary (Robson) attempt to defuse the increasing consternation of the villagers by pointing out that the letters should be ignored as the malicious nonsense they are. Their efforts meet with little success, and Rider's daughter Ann (Todd) also becomes a target with lewd accusations being made about her fiancÃ© David (Geoffrey Toone).
As the letters continue to arrive with ever more outlandish content, the social fabric of the village starts to fall apart. The letters all bear the local postmark, and people start to look suspiciously at their friends and neighbours, wondering who could be behind the campaign. Despite Rider's insistence that the contents of the letters should be disregarded, some notice that the letter-writer seems to have a very detailed knowledge of their personal circumstances, and start to question whether there may be a grain of truth in what is being written. Personal relationships too come under strain.
Soon the entire village is overtaken by suspicion and paranoia, and fingers start to point at Connie Fateley (Catherine Lacey), a shy young seamstress who lives alone and does not tend to socialise. Convinced that hers is exactly the kind of personality that would find vent in a malicious poison-pen campaign, the villagers turn against Connie, openly accusing her of being the guilty party and ostracising her from village life. Tragedy follows when the despairing Connie hangs herself from the bellrope in the village church.
Rider preaches a sermon in which he expresses his disgust with his congregation for having driven Connie to suicide without a shred of evidence against her. Most, however, believe privately that Connie's death was an admission of guilt and feel relief that the ordeal is over. But letters are soon arriving again and the police become involved, keeping watch on local letterboxes in an attempt to catch the culprit. David now starts to receive letters detailing Ann's alleged infidelity, and unstable villager Sam Hurrin (Robert Newton) is targeted with information that his wife Sucal (Belle Chrystall) is dallying behind his back with local shopkeeper Len Griffin (Edward Chapman). After drinking himself into a rage, Hurrin goes out to confront Griffin and shoots him fatally.
The police now begin a round-the-clock surveillance of all letterboxes in the village, the collected letters are analysed and everyone who has been recorded as posting a letter is required to provide the address on the envelope. A handwriting expert is also brought in. The investigations lead in a surprising direction, towards a respected community member who has managed to hide a severely disturbed mind behind a mask of caring efficiency. Realising that the net is finally closing in, the perpetrator descends into a destructive mental frenzy before fatally jumping from a cliff above a local quarry.

Question: Why did friends and neighbors suspect each other of writing the letters?
Answer: They had a local postmark.