Having seen it used elsewhere, I can honestly say that unless it was heavily DM monitored, you'd get what basically amounts to spam emails, constantly. It could be used well, and was, very occasionally, but without being very well monitored it got to the point that most players either got mad about it or ignored it.
However, I would happily support a sort of specific-location NPC that was only usable by PCs of a certain faction:
For instance, a bounty hunter crier that the Garda could place bounties on. A Morninglordian priest in training that could announce events or preach prayers... Opening it to the general public, however... Probably not so great, having seen it.