[exim] rewrite From header with smtp auth-ed username?

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Autore: krzf83@gmail.com
Data:  
To: exim-users
Oggetto: [exim] rewrite From header with smtp auth-ed username?
How to rewrite From header email adress (not whole header) with smtp
auth-ed username (like gmail does), using exim?
Alternatively denying such message would also be ok.
Please don't say it is against some rfc.