Unfair! Mail fraud was not recognised (10)

I believe the answer is:

unfamiliar

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

'not recognised' is the definition.
(I've seen this before)

'unfair mail fraud' is the wordplay.
'unfair mail' anagrams to 'UNFAMILIAR'.
But, I'm not sure how this is indicated.

'was' is the link.

Can you help me to learn more?

(Other definitions for unfamiliar that I've seen before include "Not recognised" , "Unknown" , "Strange" , "novel" .)

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

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