Spammers are always changing tactics and opening new email servers and such to try and get around the spam filters, and the spam filters have to adjust to catch up with them. My solution is to set my blog so that first-time commenters go to moderation and after that, no moderation. That keeps them from ending up on the blog, and after you mark them as spam, within 24 hours typically, Akismet will be catching them before they get to your blog.
Don't just delete spams that get through, mark them as spams so that Akismet will learn.