Obstruct the mail in capitals (5,7)

I believe the answer is:

block letters

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'capitals' is the definition.
The definition and answer can be both related to communication as well as being plural nouns.
Perhaps you can see a link between them that I can't see?

'obstruct the mail' is the wordplay.
'obstruct' becomes 'block' (blocking is a kind of obstructing).
'the mail' becomes 'letters' (I've seen this in another clue).
'block'+'letters'='BLOCK LETTERS'

'in' is the link.

Can you help me to learn more?

(Another definition for block letters that I've seen is " Caps".)

I've seen this clue in The Globe and Mail.

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