One of my “safe senders” comes thru with the address of “hotmail”

I have a friend that writes me somewhat infrequently.  They answer through their blackberry phone from work, or their own personal cell, and I'm not sure what type it is, but it's on the Verizon network.  This is what I see in my inbox: "hotmail".  This friends e-mail comes through as "hotmail" in the inbox, but, in his messages it uses his hotmail account address.  Why won't it list his hotmail address or other email address like the others in my inbox?