QUOTE(tibmail @ Aug 22 2008, 06:36 AM)

I'm not looking to appeal it at this stage as it's due to be delisted soon, I'm just trying to understand what's going on
If you mean that you didn't use the 'delist' button, that was a good decision. If you had used that and the spam continued, you wouldn't be able to use it when you get the problem fixed. It is a one time only button.
If other email addresses from the same IP address are not getting bounced, then the ISPs at the receiving end are not using the spamcop blocklist to reject email or if they are, it is only part of their filtering process. sales@ might get blocked because of the content (an obvious sales pitch or spam key words) while other legitimate emails get through the content filters or the recipient has whitelisted the addresses.
However, ISTM as if you are filtering outgoing email for spam (though why you are doing that and not monitoring it to see if spam is being sent is a bit of a mystery). If that is so, then the way your filtering system is set up, it must only use spamcop for part of the filtering process so that only obvious spam is caught.
As was suggested before, either your client has a compromised computer or they have bought a 'guaranteed all opt-in mailing list' and really are sending unsolicited email from their sales department. If someone else previously has installed an outgoing spam filter and tweaked it so that only email from sales@ gets caught, it sounds as if this has been going on for a while. You need to get the problem fixed - help your client understand either about compromised computers or best mailing list practices.
Miss Betsy