We do not provide spam protection per se. As a forwarding server if we discarded spam messages, any "false positives" that were incorrectly flagged as spam would be forever lost. That obviously won't work.
Complicating this, not all of our members are alike. They have a broad difference of opinion about what constitutes spam and/or undesirable messages. Some people provide tech support and need to be able to receive forwarded copies of spam and viruses that most of the rest of us would never want. One size does not fit all!
What we do instead is enforce strict restrictions on the email servers that attempt to send email through our forwarding systems. These restrictions are no problem for legitimate email servers (they should all be passed by default on a properly-configured system) but they represent a tough barrier for the sort of compromised, virus-infected servers that send most of the world's spam. This blocks most illegitimate messages based on delivery mechanisms, not the context of the message. If you want, you can read all about our Email Acceptance Policy.
Although a fair amount of spam slips through this net, it has been pre-screened in such a way that a client-side junk email filter, such as those offered by most email software, can rely on information about its delivery to be accurate and therefore do an excellent job of filtering the spam according to your specific personal criteria without becoming confused by all kinds of forgeries and maliciously-crafted messages.
Naturally, whatever email server ultimately handles the messages we forward for you may also impose content-based junk email filters.
The net result of this is a highly effective, layered defense that, through its dependence on client software under your control, best meets your specific needs.