how to move mail folders to time capsule?
How can I move mail folders into the time capsule and be able to access them?
You cannot use the TC as if it was local hard disk.
This is extremely risky as you cannot backup files once they have moved to your TC.
See the part here about all the issues involved.
http://apple.stackexchange.com/questions/104953/moving-individual-mailbox-to-ext ernal-drive
storing email on external drive to save space
Let me say an external drive is much easier to use than a network drive.
And be aware if you have issues with this you can easily lose all your mail libraries and there is no backup.