It seems npcmail is bugged. I'm unable to send any at all and get the standard "Oops. That command execution failed." message.
I sent it as an in-game bug (Bug report #13556)
Command npcmail
Command npcmail
[CHAT - Mayor Bryce of Bryceburgh]: It's that funny metal thing you put in your mouth, Galon, and then it goes "Boing, doing, dwang, dwang, doing, ding, boing, dwahng."
04:06:02 [CHAT - Mayor Bryce of Bryceburgh]: You know, the thing Snoopy plays to help Charlie Brown remember "i before e, except after c"
04:06:02 [CHAT - Mayor Bryce of Bryceburgh]: You know, the thing Snoopy plays to help Charlie Brown remember "i before e, except after c"