I just upgraded my MBP to an SSD and want to keep it free from data such as my email

I just upgraded my MBP with an SSD and want to keep it free from data such as my email data. I installed it in a Data Doubler from Other World Computing so I still have my 500 GB hard drive in the laptop. I want to do the same thing as I do with my music, which I keep on the hard drive.

The new solid state drive is the boot drive, 10.11.6 OS.

Is there a way I can set up mail so all my files are on the hard drive?