Maintain info when jail is in mail! (4,6)

I believe the answer is:

keep posted

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

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

'maintain info' is the wordplay.
'maintain' becomes 'keep' (synonyms).
'info' becomes 'posted' (I can't explain this - if you can you should give a lot more credence to this answer).
'keep'+'posted'='KEEP POSTED'

'when' acts as a link.

Can you help me to learn more?

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

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