Overseas post put strain on landlords (3,7)

I believe the answer is:

air letters

'overseas post' is the definition.
(letters sent by air)

'strain on landlords' is the wordplay.
'strain' becomes 'air' (synonyms).
'on' means one lot of letters go next to another.
'landlords' becomes 'letters' (people who let property).
'air'+'letters'='AIR LETTERS'

'put' is the link.

I've seen this clue in The Guardian.

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