Yes, it should be possible to run an email server as a Sandstorm grain. Note that you would need to grant the grain networking capabilities so that it could talk to the outside world. Currently, only the admins of a Sandstorm server are allowed to grant such capabilities, but that will change once we've implemented our plans for trusted "driver apps" that attenuate and manage admin capabilities.

A grain can keep itself running in the background by requesting a wakelock from its Sandstorm supervisor process. While the grain holds the wakelock, it does not get automatically spun down, even if there are no users visiting it.