I'm using Bruce's book too. I wrote to Dropbox and here is their reply:
Thanks for writing in to Dropbox Support! I'd be happy to answer your question today.
We occasionally use certain Amazon domains (in this case, amazonses.com) to handle various kinds of data. Unfortunately, due to the nature and design of DMARC and specifically DKIM, the only way to work around this is to disable verification.
I hope this information helps. Please let me know if I can be of further assistance.
Obviously not much help, they don't even seem to care--just 'turn it off'.
Bruce, the sender information is included in the log above. The domain that is rejecting their email is firstname.lastname@example.org (or any other domain hosted on that server) MX for the server is email.dandylionhosting.com.
Sorry for my late reply, I've been out of town and I actually did turn it off so that a couple of clients could get email from dropbox.com while I was gone. Now that I'm back, I don't want to leave it turned off, so there has to be something that can be done to fix it, I would hope.
If you need any other info, just let me know.
Thanks again to all that replied. It's greatly appreciated.