concering dovecot logging

hi

had a question concerning dovecot lda logging.

deliver_log_format = From:<%f>-<%e> :: Subject:<%s> :: Status:<%$> :: MsgID:<%m> :: Size<%p> :: vSize<%w>

am getting %e ie envelope sender as MAILER-DAEMON in the logs

Sep 11 08:35:50 lda(noc@xxx.com): Info: sieve: From:bmmsro0612@xxx.co.in-<MAILER-DAEMON> :: Subject: :: Status:nocforward@yyyyy.com> :: MsgID:<OFBECDF3FA.57B3772C-ON65258198.00111392-65258198.00111394@xxx.co.in> :: Size<12497> :: vSize<12687>

what could be the reason ?

thanks, rajesh