What E-Mail Validation Tools Do You Use?

morcego asks: "As we are all too much aware, spam is an increasing problem. Each of us has our own set of tools and methods to try and reduce the amount of spam we receive, each with different pros and cons. Also, on a more broad front, we have options like SPF (+ SRS), Microsoft's own Caller-ID, and Yahoo's DomainKeys that we can use. These days, it is incredibly easy to implement any (or all of these), using publicly available frameworks and libraries (libspf2, and milter, to name a few). I have been using SPF for quite some time now with some measurable results, although nothing earth shattering. Which of these are you using, if any? Why, or why not? Do you think any of them really contribute anything to fight spam?"