I think I'm being really thick here.....Does your spam on the iPhone download into a 'Spam folder' or does it download into the 'Inbox folder' and mix up with your legit mail?
It's all a case of WHERE the spam is being filtered. It's obvious that his BT account has no filtering inherent on the server, hence when he downloads it all to Outlook, Outlook does the filtering on site and hence moves them to a local junk mailbox. The iPhone doesn't have a junk mail filter per se, so that is why no on-site filtering is performed. In fact it's not atypical to have a mobile device with no form of inherent spam filter.
The *best* method therefore is to have the spam filtered out at the server end - i.e. BEFORE it gets downloaded to your Outlook or your device. This is why reputable IT companies always employ spam filters at the server end
