Do you have Facebook contacts turned on? It may be at some time you got an email message from them and that is the way they have their return address set. All previously used addresses are in a cache on the phone and the only way to clear that is to restore the phone as a new device, which deletes all data from the phone. You cannot restore from a backup because the information is in that and will just be put back on the phone.
I don't have them turned on. The cache you mention is probably my issue. My phone picked up some contacts from my wife's address book on the cloud one time. I turned it off later and now only sync with exchange contacts but I guess it retained some of that info. Sure hate to start over with my phone.