If you're talking about email specifically, and if the email server has DKIM, and if the keys are secure (if the hacker can break into the server to steal emails, are you sure they didn't steal the key too?), then yeah. But many document dumps include a lot of stuff that isn't just emails, and many email document dumps probably aren't fully covered by DKIM (I say probably because I haven't checked, nor am I really in a position to do so) or the DKIM key may not necessarily be secure (if the email server itself is hacked, it may not be; if the user login to the email was hacked, presumably it will be).