The email came from Gumtree. So yes any man, dog, thing with eyes could have seen it. Also, I think Gumtree make you reply to emails through Gumtree for added safety. I know that when I used to reply to people on gumtree I was shown their email address. This is no longer possible, from what I can tell.
No email address provided, which I know sounds odd, but it came through Gumtree and for your protection they make you reply through the site rather than handing out email addresses, if that makes sense?