It's difficult to know what is happening without being able to see your config files but if it is a specific email address you are trying to whitelist I recommend putting it in /etc/virtual/whitelist_senders
I would look for the address being blocked in /var/log/exim/rejectlog to make sure that the address you place in whitelist_senders matches exactly.
If that doesn't work then it would be helpful to see the line in your log file where the email is being rejected and the contents of the applicable whitelist file in /etc/virtual and possibly the contents of your exim.conf file.
Roark