Amazonon DMARC (http://sesblog.amazon.com/post/Tx22ZELXSSZRYZR/What-is-DMARC-and-should-you-use-it).

This Google article is also helpful: https://support.google.com/a/answer/2466563?hl=en

If you scroll down to the ‘Example records’ section, you’ll see a number of examples on how to write the TXT record. Here’s an example of that record:

_dmarc.advancedmediawebs.com. 3600 IN TXT “v=DMARC1; p=quarantine; pct=5; rua=mailto:mark@domain.com”