Depending what e-mail server you use. If there's a way you could look up ALL mail including the spam. There is sometimes a button you can click on that says. Not spam click back to inbox. G-mail and yahoo and some others usually have a way you can get back what you clicked into spam so you can save it without the next mail coming from that person going into the spam box automatically.
I only know this because I use G-mail and it has that option.