"No junk mail"

I occasionally distribute flyers in our area regarding activities at the Church.  We're always told, don't put one in any letterbox marked "No junk mail".
But what exactly is "junk mail"?   It reminds me about one person who complained that she never received the local paper.   When asked, she explained, yes, she did have a "No junk mail" sticker on her letterbox.   But, to her, the local paper wasn't "junk".

True, some signs say, "No advertising material".  I think this is intended to convey the message that the local paper is OK.  But I don't know whether a flyer produced by the Church giving details about the times of services over the Christmas period is "advertising material", so I take the cautious approach and give these letterboxes a miss.