Users of mail placards (7)

I believe the answer is:

posters

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'users of mail placards' is the definition.
'posters' can be an answer for 'placards' ('poster' can be a synonym of 'placard'). I am unsure of the 'users of mail' bit.

'mail placards' is the wordplay.
'mail' becomes 'post' (synonyms).
'placards' becomes 'ers' (I am not sure about this - if you are sure you should believe this answer much more).
'post'+'ers'='POSTERS'

Can you help me to learn more?

(Other definitions for posters that I've seen before include "Public placards" , "Bills or placards on walls" , "Contributors to forum" , "Placards, notices" .)

I've seen this clue in the Sydney Morning Herald.

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