Pipe cor

In the dovecot-documentation, there are exim-variables used in the pipe command (http://wiki2.dovecot.org/LDA/Exim). To make them work, we need to expand the expression obtained from the database. One could directly write the users-address but it won't work for the $sender_address:
/usr/local/libexec/dovecot/dovecot-lda -d $local_part@$domain -f $sender_address