I added a note about the problem in the wiki! Regarding the documentation, I think there are two places where this problem is worth mentioning:
- Mike Perry's blog entry regarding minimal harassment [1], bullet point #3. - The documentation page regarding manual relay setup [2], step #4, bullet point #9.
For both pages, here is what I would mention: "If you set up a reverse-DNS for your exit node and allow SMTP in your exit policy, you should consider configuring SPF (https://en.wikipedia.org/wiki/Sender_Policy_Framework) for your domain. If you don't, spammers may use your relay to convincingly spoof e-mails originating from your domain."
Possibly adding a link to the original discussion [3] afterwards, if the policy of the documentation allows external references. Let me know if this is clear enough for you. I can also write a page dedicated to this particular problem in the wiki, if you think it's worth it.
[1] https://blog.torproject.org/blog/tips-running-exit-node-minimal-harassment [2] https://www.torproject.org/docs/tor-relay-debian.html.en [3] https://lists.torproject.org/pipermail/tor-relays/2014-August/005168.html
-- JusticeRage