blacklisting

A guy here at work says he is unable to send email to his wife at work.  She
said her network people say it's because our site is listed somewhere as
sending spam.

We don't send spam.  A few years ago we got blacklisted because we were an
open relay.  David helped me fix that.  How do we find out if we're listed
anywhere?  And how come that's the only place (that I know of) where we're
having a problem?

Thanks.

(David, if this is the wrong list, tell me where to post it.)