As Radim noted, there might be better approaches to using SVMs. One example may be Restricted Boltzmann Machines (see Hinton's google tech talk [0]). Some folks have tried using them to detect spam (or at least using RBMs as a part in another architecture), achieving better results than SVMs (they actually did a proper comparison). [1] [2] Might be something worth to be looked at ;) At any rate, RBMs are rather fascinating, I also plan to try experimenting with them when I have time.

[0] https://www.youtube.com/watch?v=AyzOUbkUf3M

[1] For a general overview of the study, here are its slides (pptx): http://users.cs.uoi.gr/~gtzortzi/docs/publications/Deep%20Be...

[2] The same study in full (pdf): http://users.cs.uoi.gr/~gtzortzi/docs/publications/Deep%20Be...