AIM Surplus is a premier provider of gun parts and gun related stuff. Have you or someone you know ordered parts from them recently? It appears that somebody compromised their site and stole information from FFLs and individuals with C&Rs who have them stored there for repeated orders?
AIM is kosher, but the letter may not be. Contact them directly to determine if they actually sent this. It may be a mining attempt to get information from you.
http://www.aimsurplus.com/