Estimates say that spam makes now up for 80 - 90% of all emails, and many mail servers have difficulties in managing the additional load caused by the latest spam, and spam filters such as SpamAssassin do not recognize large parts of that spam as they did before. Fortunately, we can block a big amount of that spam at the MTA level, for example by using blacklists, running tests on the sender and recipient domains, etc. An additional benefit of doing this is that it lowers the load on the mail servers because the (resource-hungry) spamfilters have to look at less emails.