filter messages on the server

I use my Mac to download my e-mails when at sea using a sat phone. However, with SpamSieve I have to download the whole message (be it either spam or good) to then apply the filters. By the time the message is in the computer, I have already waisted a lot of time/bytes and money downloading spam messages (Sat phone communications are expensive and slow!). Entourage 2004 (and perhaps other versions too), has a possibility of “Allow online access (shows account in folder list)”. This possibility shows the messages that are in the server with its sender name, subject and if there are any attachments. Is there a possibility to have SpamSieve apply its filters to this folder (the account folder that is shown in Entourage) so that I can rapidly choose which messages get deleted “before” they are expensively and slowly downloaded to the Mac?

Yes, there’s no way around that. SpamSieve needs to see the entire message in order to decide whether it’s spam. Filtering the messages based just on the sender/subject/etc. would be quite risky. If you have a second Mac that you could leave at home, one option would be to setup a spam filtering drone. Then the Mac with the faster connection could download the messages and filter them with SpamSieve, and your Mac with the satellite phone would simply ignore the contents of the Spam mailbox.