This can be done, but is not recommended due to network latency frequently seen on NAS devices. If this has been accounted for it should work pretty well though. SmarterMail can suffer during mail spooling if IOPS climb too high (disk latency becomes extreme) which can lead to the server eventually failing to spool mail as these back up in the queue. So, if this NAS is backed by a fiber connection or similar you can likely get away with this. I'd recommend deploying in a dev environment first so you can complete load testing on it before rolling it out to production.