Set Sieve forwarding sender to the recipient

This should allow forwarding via Sieve scripts to work with
SPF (otherwise emails will likely bounce).

roles/mail/templates/dovecot/conf.d/90-sieve.conf

@@ -176,7 +176,7 @@ plugin {
   #
   # This setting is ignored when the envelope sender is "<>". In that case the
   # sender of the redirected message is also always "<>".
-  #sieve_redirect_envelope_from = sender
+  sieve_redirect_envelope_from = recipient
 
   ## TRACE DEBUGGING
   # Trace debugging provides detailed insight in the operations performed by