Keep informing jail in the mail (4,6)

I believe the answer is:

keep posted

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

'in the mail' is the definition.
I can't judge whether this defines the answer.

'keep informing jail' is the wordplay.
'informing jail' becomes 'posted' (I am not sure about this - if you are sure you should believe this answer much more).
'keep'+'posted'='KEEP POSTED'

Can you help me to learn more?

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

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