Can I save Mail to an external drive?

Is there any way that i can move all downloaded mail from the Mail App to a different folder on my external drive? I just downloaded DaisyDisk and can see that the Mail folder in my Library is taking up over 12GB! I have an SSD too, so space isn't so easy to come by on my internal drive...
What i would ideally want, is for mail downloaded within a month to stay on my internal, and then delete itself after a month of being opened and read. So, in this time from opening i would have my mac automatically (or i would manually do it, if i had to) save that months mail to my external drive.
Any ideas anyone? I realise its a bit of a longshot, as Mail isn't as easily configurable as say iTunes or iPhoto, but any help would be much appriciated.
Thanks