Add O option to dovecot transport in postfix master.cf so that X-Original-To: header is added.

From:

dovecot unix - n n - - pipe flags=DRhu user=vmail:vmail argv=/usr/lib/dovecot/deliver -f ${sender} -d {user}@{nexthop}

To:

dovecot unix - n n - - pipe flags=DROhu user=vmail:vmail argv=/usr/lib/dovecot/deliver -f ${sender} -d {user}@{nexthop}