1). You see a letter box with a sign saying "No junk mail". You avoid it. Later when canvassing the occupant complains they never get anything through the letterbox and says that their sign is just to stop leaflets about pizzas, doubleglazing, taxies and so forth.
2). You stick a leaflet in the letterbox and someone subsequently comes out to complain (or sends a complaint to party HQ).
Both scenarios can cause difficulties, but in the second case the person in the home is wrong because election & political communications are not junk mail, i.e. commercial advertising, but rather "informative" material. If people put up "no leaflets" or "no circulars" (or even the more specific "no political/election leaflets") then the situation is very different. This is the definition distinction in all the formal advice I've been given over the years, but it can confuse when people put up instructions without checking the terms used. See for example Stop Junk Mail: Political junk mail, and how to stop it which has the right idea but continues the confused use of the term. See also ft.com/westminster: Our junk mail isn’t junk mail.
For those who have been on the receiving end of angry recipients, here's an interesting story from an Official Monster Raving Loony Party leaflet:
Election Leaflets Are Not Junk Mail
Earlier in this campaign, I was confronted aggressively by an irate person who objected to the fact that I had just delivered a election leaflet through his letterbox. This was despite the fact that his notice merely said "no free newspapers or junk mail" but it did not say "no leaflets" or "no circulars".
I politely explained to him that election leaflets are not junk mail (junk mail means advertising such as pizza leaflets), but he mad a specific and overt threat that he would "punch me in the mouth" if I continued the conversation.
I immediately went to the police station and reported this threat, but I was told that the police would not bother to do anything because it was "not a arrestable offence". Such threats are unacceptable in a democratic society. Those of us - candidates and agents - who are engaged in the legitimate business of communicating our policies to the electorate during a election campaign need to be confident that we will be protected by the law if necessary.
The full leaflet can be seen at Official Monster Raving Loony Party: Croydon Branch: ELECTION LEAFLETS part 1: 2005 to 2010.
The campaigner subsequently produced a limited print run second version of the leaflet in question, this time including the above text, and delivered it to the same household to explain the point to the voter directly.