Hell Correspondence is a website used to summon Hell Girl to seek vengeance on those who have wronged them.
Hell Correspondence is a website only accessible at midnight in Japan to those looking for revenge on others who have wronged them. One must wait until midnight on the dot before getting to the main page. Once there, they must type the name of their transgressor into the box and click send. Afterwards, Ai Enma receives the message and meets whoever it was that summoned her. However, it has been shown that Ai reserves the right to either ignore requests that are faulty, are not true grievances, being forced by someone else to access the site, or requests that can not be achieved. Ai also denies requests when the person is being tormented but does not know the person's name (Note: It depends on the circumstances. For example, a girl who was being harrassed by a stalker, did not know the name, but was granted her request. Another instance, a girl was being bullied but did not know who it was. Each time she typed in a classmates name, it was denied since it was her teacher). Afterwards, Ai sends a text message saying the request has been accepted. When Ai does meet with the requester, she introduces herself as she bequeaths on to them a straw doll, Wanyuudou, with a red string around his neck. Ai then explains how the service works: pull the red thread to send their enemy to hell but doing so binds one into a pact that when they die, they go to hell as well.
Certain times, Wanyuudou, Ren Ichimoku, and Hone Onna transport the requester to the Realm of Eternal Twilight to hear more about their grievance. Other times, they will scope out a situation such as keeping an eye on both the aggressor and the requester, that way they can assess the situation and be there once the thread has been pulled. On very rare occasions has someone contacted the Hell Correspondence, received a doll, but decided not to pull the thread. In cases such as that, the Hell group returns to the Eternal Twilight; this does not mean the person who decided not to pull the thread can not contact Ai again.
Something to note is that the Hell Correspondence has existed for nearly four hundred years, dating back to the Warring States period in Japan. When new technology emerges, Ai and her associates find new ways to communicate with people. For example, in the 1950s, Hell Correspondence was in the Missing Persons column in the newspaper but only those with a strong hatred could see it; to others, it was a blank square. A man who used it then, Fukumoto, states that Ai was summoned via delivering a letter to a mailbox late at night with his request as she appeared with the straw doll. It has only become more well known in the digital age due to information sharing and word of mouth.