Rewrite envelope from address in sieve forwards (#1949)

Fixes #1946.

setup/mail-dovecot.sh

@@ -183,6 +183,7 @@ plugin {
   sieve_after = $STORAGE_ROOT/mail/sieve/global_after
   sieve = $STORAGE_ROOT/mail/sieve/%d/%n.sieve
   sieve_dir = $STORAGE_ROOT/mail/sieve/%d/%n
+  sieve_redirect_envelope_from = recipient
 }
 EOF