[exim] per-mailbox hold_domains

Hi all,

local_part isn't available at the time hold_domains is expanded. If
there a way to achieve something like this:

hold_domains = mysql;SELECT SUBSTRING_INDEX(address, '@', -1) AS domain \
                   FROM mail_hold \
                   WHERE address =
'${quote_mysql:$local_part}@${quote_mysql:$domain}' \
                   OR address ='%@${quote_mysql:$domain}';

Thanks,

Ian