I don't have much knowledge of the subject but as I know, when servers receive an email they have two sources of data to classify it as SPAM or not. The contents of the message (subject + body) and the metadata that goes with it (from who / to who / server that sent the message, etc).
I know you can do basic filtering by blacklisting emails list and other sorts of metadata. But is it possible to do decent spam filtering without checking the contents of a message? Defining 'check the contents' as using any function that relies on data found on the body/subject of the message to classify it as spam or not spam.
Edit: The reason I ask for this because of the "scroogled" campaign where Microsoft accuses Google of scanning the contents of emails for advertising purposes, and I think that they do the same for spam prevention purposes. So I asked them and they said:
We have tools and system set up to filter spam emails to and from Outlook.com email accounts. However, we are not allowed to disclose how these spam filters work for security purposes.
ME: So can you assure me that no content from my personal email is being analysed and cross referenced with other data in any way?
...In response to your question, its a yes, we can assure you that.