I'd generally let something like this slide if it was a long time ago and has nothing to do with what someone is doing now.  For instance, if he ran a mail fraud 20 years ago and is now accused of robbing a bank, it isn't all that relevant.

However, when someone is running what looks awfully like the very mail fraud they were recently in prison for perpetrating, I am less inclined to cut slack and more inclined to think a leopard doesn't change its spots.