another configuration of gone-away

This is another configuration for handling removed accounts. Thanks
to Nick Waterman <nick@???> for the suggestions.

I found out that 'once' can include '${local_part}'. I think that's
pretty handy.

Note that the 'user' specified below must have write access to the log
file and 'once' directory.

gone_away_transport:
driver = autoreply;
from = postmaster,
to = ${reply_address},
cc = postmaster,
log = /usr/local/lib/exim/gone-away/gone-away.log,
once = /usr/local/lib/exim/gone-away/${local_part}.once
return_message,
subject = "Invalid Account",
text = "${local_part} no longer works at COMSAT.\nPlease remove ${local_part}@${domain} from your mailing-list.\nPlease contact postmaster@??? if you require more information.",
user = exim,
end

gone_away:
fail_verify,
driver = aliasfile,
transport = gone_away_transport;
errors_to = postmaster,
file = /usr/local/lib/exim/gone-away/gone-away.list,
search_type = lsearch