Crossword Genius

Said about half of the population's posts (5)

Ross

I believe the answer is:

mails

I'm a little stuck... Click here to teach me more about this clue!

'posts' is the definition.
(I know this)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for mails that I've seen is " Sends by post".)

I've seen this clue in The Independent.
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