[exim] X-Loop headers

I am trying to add X-Loop headers to vacation replies.
My vacation transport:

vacation_autoreply:
driver = autoreply
to = ${sender_address}
from = "${local_part}@${domain}"
log = /var/log/exim/exim_vacation.log
once ="/localdrive/$local_part-vacation.db"
once_repeat = 7d
subject = "Re: $h_Subject: from ${local_part}@${domain}"
headers_add = X-Loop: "$local_part@???"
text= ${lookup mysql {MYSQL_VACATION}{$value}}

No luck so far. Any suggestions?
/sd



--
Steve Devine
Email & Storage
Academic Technical Services
Michigan State University