Yet such mail does not consist of such letters (5)

I believe the answer is:

chain

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'letters' is the definition.
(chain letter is a kind of letter)

I can't explain the rest of the clue.

(Other definitions for chain that I've seen before include "Fetter" , "Concatenation" , "Mountain range" , "Wickets so far apart in cricket" , "number of shops" .)

I've seen this clue in The Globe and Mail.

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