Then your cache is lost on every reboot... Evolution (mail client) by default stores fetched email in there (I moved it to other place for backup purposes). It'd suck to have your mail client re-fetch emails every time.
I use webmail, so haven't had any problems. Seems like something under `$HOME/.local`, or maybe its own dotdir, would be a better place for downloaded messages.
How often do you reboot linux? I go months between reboots, and that's with crappy hardware.