Such letters do not form such mail (5)
I believe the answer is:
chain
'mail' is the definition.
(synonyms)
'such letters do not form' is the wordplay.
I cannot quite understand how this works, but
'form' could be 'chai' (I have seen 'Form of tea ' mean 'chai' so perhaps 'form' could also mean 'chai') and 'chai' is found within the answer.
A single letter 'n' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'such' acts as a link.
Can you help me to learn more?
(Other definitions for chain that I've seen before include "Paper decoration" , "British biochemist identified?" , "Series of connected links" , "Succession" , "Connected sequence" .)