The 'Who the hell are you?' conundrum.
-
I get where this is coming from.
I've actually been the "Bad guy" in this regard before - but I'll run down how I tend to handle letters to NPCs.
Do I just happen to know this character? - If so, did I, personally, directly interact with them using this NPC? if so, easy to respond in a totally direct IC manner - But, check the DM forums for notes on the player/this NPC, to see if they NPC recently got possessed by an evil Half-Moloch or something, or recently had a fight with this character - Adjust response based on these potential changes.
If I don't happen to know this character -
Search through the DM forums (we have sections like - PC notes - Player posted - and PC notes - DM posted. We also have places like NPC notes, and then the applications forum) so I check those first to see if this character has been mentioned and if I can get any info.
If I can find nothing, this is where it get's complicated. Hopefully, in the player notes - PC/DM posted sections, or applications, there was SOMETHING about this character. Otherwise, I have... very, very little to go on.
I tend to base the level of "Fame" (this is my own personal way of handling things regarding NPC reactions to PCs) on how much literal buzz there is about characters in game actions. I might breifly check the bulletin boards, check the militia/other faction forums (can you see how this is taking a LONG ass time for one letter yet?) to see if I can figure out who in the game world does know them. I may even look at the IC letters forum for clues, and potentially, if I've really got a whole lot of spare time lying around, I might even check the auction board. It's possible this player character is actually selling something this NPC wants! or, maybe they are a well known smith, with epic skillz, and this NPC respects/hates craftsmen?
If after having exausted all of these things (assuming I had the time to check through all of this) I can find near nothing to guide how this interaction should go. I will check the NPC in question again, to make sure I have their general personality right, and try to respond in a way that makes sense for them. Often this will be something like - "Come back and ask when you've made a name for yourself" or a slightly more helpful "I don't really know who you are (OH MY GOD LOOK I JUST DID IT!!!) but if you can solve (insert problem here) for me, we can talk.
I mean, there isn't THAT much more I can do, without any actual information.
Is there?