This discussion shed light on the policy a lot of newspapers have that anonymous sources can't be used in stories. Let's say, for example, an anonymous tipster calls an editor and accuses a mayor of beating his dogs nightly.
If the newspaper runs that charge an attributes it to no one, then it could get in trouble should the mayor sue because he doesn't beat his dogs. If the newspaper tried to claim it was simply repeating something from an anonymous tipster, then it can be argued the newspaper made up that tip and is simply trying to pass along the blame.
However, if say the city clerk calls in, makes that claim about the mayor and is identified in a story, that's another situation entirely. The blame can shift to the city clerk in a libel suit.
Still, to be completely secure, a newspaper with such a tip should call the mayor directly, get his side of the story and print that, too. A newspaper does have some duty to investigate and determine if there is a factual basis supporting the articles it publishes.