Functionally similar to the word unacceptable but with added sarcasm. Suitable for use in mass emails.
Originally employed to decry some action or phenomena while displaying a degree of carelessness indicating that the speaker is more interested in the act of denigration than the subject of it. However, it has grown to be used primarily as an indicator that no disdain is implied.
"The office is unlocked with the lights on and nobody home... This is UNACEPTABLE!."
Dude1: "Dude, here's that sandwich you wanted. Don't get used to this, I'm not your man-servant."
Dude2: "Dude! Does this have mustard on it?!"
Dude1: "Uhhhh, yeah ... I guess"
Dude2: "This is UNACEPTABLE!."