Specifically for PeterM's library situation, though, it may have been from an "unknown" email address. At least, I know that I did virtual reference while I worked at the library, and the person is basically online chatting with you, and then they get a follow-up email sent to whatever email they logged on with. But I'm not sure it would be clear, or that I'd even know what to tell the patron if they asked, for what email their reply would come from. At best I might be able to give them a domain, in case they could put that domain on their safe list. I don't know if Peter uses the same system, though, and this was also when I worked at the library back before my oldest little penguin was born, some 6 years ago, when it was fairly new.