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

If you are using Time Machine your Mail is already backed up and stored on an external drive just high lite the mailbox you want then enter TM and go back to what you need.