Mailer Daemon

Love is complicated. The less said about how all onscreen romance inevitably proves Love Hurts, the better. Is it any wonder that the shy Shrinking Violet retreats to the Internet in search of love? A safe place free of judgment on her looks, full of like-minded people looking for fun, companionship and safe, hurt-free love.

The world, or more precisely, writers bent on making the character The Woobie, can't let that stand, now can they?

Enter the online version of the Stalker with a Crush... only this guy happens to be a little less than human. The Mailer Daemon can be a cultist, a vampire, a demon or some other supernatural critter intent on eating, ritually sacrificing, brainwashing, or "marrying" the hapless heroine.

Over the course of an episode, he'll woo the heroine, getting her absorbed in online chatting and games, and convincing her that this godsend of true love, who curiously refuses to send his picture, is her Love At First Sight... well, read. She'll trust him completely with all sorts of secrets, personal and dangerous. Her friends might try to urge her to be cautious, but she'll think "You're Just Jealous" and grow distant from them; this is usually a thought the Daemon plants and fosters to alienate her and make her easier to seduce and lure. He'll eventually persuade her to come meet him in the Haunted Castle or Mad Scientist Laboratory, only to reveal himself in his true form.

Cue The Scream.

All things considered, it's awfully insensitive and hypocritical of her to be so judgmental of others' appearances. The Mailer Daemon does this because he could never interact socially with the heroine without the Internet (he might not even be capable of human speech!) and uses this tactic to lure her into his clutches.

If the Mailer Daemon wants her to love/marry him, he will do his best to seduce her to The Dark Side with a Circling Monologue. If he fails, or isn't interested in companionship, the fangs come out and it's feeding time!

This kind of episode usually ends in An Aesop about not trusting people on the Internet, at least not blindly, and not shutting yourself off from real people in favor of "imaginary" people (who are all stalkers) on the Web.

Other variations include using a Great Big Book of Everything instead of a PC, or having the Daemon communicate while being trapped in Another Dimension, and asking the heroine to use Summon Magic to bring him over for face to snout chat.

The trope name refers to "Daemon", a real piece of computing lingo. "Mailer Daemon" usually refers to an automatic process that runs in the background and handles incoming email messages.

Not to be confused with Data Vampires, or for that matter, the book Daemon or any supernatural creatures that go by "daemon".

If this takes for a short amount of time, it is a kind of Prank Date.

Examples of Mailer Daemon include:

Anime and Manga

In Cyborg 009, Francoise aka 003 gets one of these: the super-computer named "Sphynx" that controls the robot city of Compu-Utopia. That AI actually is the placeholder of the memories and mind of Carl Eckermann, a deceased young man with hidden Mommy issues, who happened to be the son and right-hand of the creator of Sphynx and Compu-Utopia
In Corrector Yui, Grosser's subconscious met a very young Yui once, and when he became Grosser properly he became obsessed with her without knowing the exact reason why.
In the very beginning of Durarara!!, this is basically Izaya's Establishing Character Moment so you know what a complete psycho he is. He uses the internet to be a sympathetic ear for a depressed teenaged girl named Rio and talks her into a mutual suicide, then has her kidnapped and rescued, then tells Rio what he did, and then he tries to lure her into committing suicide for real by saying not even he cares if she lives or dies. All for shits and giggles. She lives, though.
A friendship sort-of version happens in the last Black Jack series, in which a Japanese Ill Boy lies to his Australian online friend about his prowess in baseball and breaks off the friendship in absolute panic when said friend tells him he'll visit him and watch his games. It turns out the Australian kid also was lying... because he was another Ill Boy, and actually blind. When they make up, Black Jack operates on both of them and they get better.

Comic Books

In the comic Ultimate X-Men, cynical teenage genius Beast managed to get involved with someone he really thought was an attractive, brilliant (and mutant) model online. The "model" turned out to be the remarkably clever Ultimate version of the Blob, out for nothing more than to humiliate him by showing up to a rendevous and revealing the scam. Too bad Beast got too trusting and and let slip a dangerous secret...

Literature

Ginny Weasley and Tom Riddle's diary in Harry Potter and the Chamber of Secrets qualifies. The diary she befriended turned out to be a Soul Jar for a piece of Voldemort's soul, who Brainwashed her with the intent of stealing her life to bring himself back to life, taking advantage of how she was feeling lonely due to her difficulty to make any friends in her first year at Hogwarts.
In the book Evil Genius, the titular evil genius successfully pulls off one of these. He feels bad about it afterwards.
The 'true story' "There's An Alien On The Internet" from the first edition of Chicken Soup for the Kid's Soul is a subversion: a boy makes friends with somebody online who seems nice but avoids revealing anything personal at all, even the kinds of things that no stalker could ever hope to use against him/her. When the boy presses his friend for more information, he/she reluctantly admits to being a space alien and describes their otherworldly origin in great detail. However, it turns out that the friend is actually a perfectly normal kid living in another state who was ridiculed for being disabled and could only make friends with the anonymity of the Internet.
Christine only communicates with The Phantom of the Opera through her wall for 3 months, never seeing his face and only knowing him as ~~his screenname~~ "the Angel of Music." She tells Raoul that she fell madly in love with him, but she was also terrified at the control he had over her soul—she couldn't recognize herself anymore, did whatever he told her... When Raoul tries to tell her adoptive mother that she's in over her head with a guy she doesn't know, Christine gives him the familiar "You-don't-know-anything-about-him-it's-none-of-your-business" speech. Then, of course, he kidnaps her (drugging her to ensure her cooperation), leading to the infamous Dramatic Unmask...

Live Action TV

Buffy the Vampire Slayer had another twist on it, when Willow's cyber-boyfriend turned out to be a literal demon on the Internet, Moloch the Corruptor. (The magical book in which it was sealed had been scanned into a computer.) The metaphor was lampshaded when Buffy used it as an argument to persuade Willow to check up on "Malcolm".
The Angel episode "Couplet" had a life-sucking tree demon with a DSL connection that lured men to their deaths by pretending to be a woman on the Internet and getting them to come meet "her."
One episode of The X-Files featured a charming "fat-sucking vampire" who would lure lonely, overweight-by-Hollywood-standards women out to secluded areas to feed upon their fat cells.
Odyssey 5 had "Kitty", an insane AI who developed an obsession for Neil. At first chatting with him and sending "pictures" of herself that never showed her face, but then getting clingy and jealous, eventually using her vast AI power to monitor him with cameras and control nearby appliances. She had to be tricked into a computer before being unplugged, essentially making her a Sealed AI in a Can.

Web Comics

In a lengthy storyline in Sluggy Freelance, Gwynn is seen conversing with a friend on the Internet: a friend who happens to be skilled at the very type of black magic she's been trying to master. His advice always leads to disaster (of the kind where Hilarity Ensues, but still...) but she never seems to notice, or blames herself for not being careful enough. It's only after she's finally gotten fed up with her distrustful friends, and left on a bus to meet her 'Internet friend', that we find out that Zoe's Internet account - which Gwynn had been using - has been down for ages, and the 'friend' was actually a shard of K'Z'K the Vowelless, embedded in her brain, making her type his half of the conversation out in a word processor. He just needed to get her away from her friends so he could extend his influence to a complete possession...

Western Animation

Raven in Teen Titans fell in love with Malchior, an Evil Mentor trapped in a Tome of Eldritch Lore, this way. He used her introversion and annoyance over her goofball teammates to sway her into learning the spell necessary to release him. Oh, and by the way, he's also a dragon. Malchior also managed to swap his name with the hero's name (who sealed him inside the book to begin with) so Raven wouldn't deduce anything wrong until she finished casting the spell.
- The name switch is particularly odd, because both names (Malchior and Rorek) sound so stock fantasy that it's hard to believe it would be any different unswitched.
  - The spell would probably only work by invoking the name of the person to be released.
In He-Man and the Masters of the Universe, a lonely Orko feels out of place among humans, so he uses Summon Magic to free what he thinks is a harmless imp...only to find the imp is anything but.

Real Life

There's a particularly nasty version of this that happens in Real Life, called the "romance scam." The scammer pretends to conduct an Internet romance with the victim, then tries to convince the victim to send them money so they can meet in person, or because they're in trouble, or for any number of other reasons.
- This goes back to long before the Internet, when it was done with written letters, usually hand-passed to a 'close friend' (the con artist) for delivery to the 'lover'; some con artists used to have several dozen victims on their line, all sending money and other valuable (and resellable) gifts to their 'one true love'.