The most common and most likely answer to this crossword clue is the 7 letter word POSTERS
I've seen this crossword clue in The Guardian
P
O
S
T
E
R
S
More clues leading to the result POSTERS
Buddy explains!
The straight part
We know that "mailbox users" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mailbox users = posters.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does POSTERS mean?
POSTERS noun- A picture of a celebrity, an event etc., intended to be attached to a wall.
- An advertisement to be posted on a pole, wall etc. to advertise something.
- One who posts a message.
- A shot that hits a goalpost instead of passing into the goal.
- A posthorse.
- A swift traveller; a courier.