RBLS is ineffective for a single email address

I have RBLS option set in Main >> Service Configuration >> Exim Configuration Editor and it works fine in that about 70% of all email coming in rejected as spam with the message "JunkMail rejected - x.x.x.x is in an RBL, see http://www.spamhaus.org/query/bl?x.x.x.x" with almost no false positives.

This works for all domains and all email accounts on those domains with the exception of a single email address.

The email address in question receives about 100 emails a day of which 90% is spam and there is not a single rejection in exim_rejectlog for this email address. More strangely, sometimes a sender in RBLS fires a dozen emails to various addresses on that email's domain. All but one that is addressed to that single address are rejected and recorded in reject log while that single address is not even checked against RBLS list.

I tried deleting the account in question and re-creating it with no change. Did anyone come across anything like this? Can you give any pointers as to what may be going wrong? Thanks.