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'Yahoo! Breaks! Every! Mailing! List! In! The! World!' says email guru

Purple Palace tweaks security setting, sparks worldwide email bounce blizzard

Internet Security Threat Report 2014

Email luminary John Levine has accused Yahoo! of sabotaging email lists for everyone, everywhere.

In a post titled “Yahoo! Breaks! Every! Mailing! List! In! The! World! Including! The! IETF's!'”, Levin explains “an emerging e-mail security scheme” called DMARC that “lets a domain owner make assertions about the From: address, in particular that mail with their domain on the From: line will have a DKIM signature with the same domain, or a bounce address in the same domain that will pass SPF [sender policy framework.”

Levine explains that DMARC has weaknesses, notably because “Lists invarably [sic] use their own bounce address in their own domain, so the SPF doesn't match. Lists generally modify messages via subject tags, body footers, attachment stripping, and other useful features that break the DKIM signature. So on even the most legitimate list mail like, say, the IETF's, most of the mail fails the DMARC assertions, not due to the lists doing anything 'wrong'.”

Most of the time that's not a big problem for the world at large. But Levine says “over the weekend Yahoo published a DMARC record with a policy saying to reject all yahoo.com mail that fails DMARC.”

Aside from lots of bounced emails that should go through, here's what Levine says will result from Yahoo!'s change:

“Since Yahoo mail provokes bounces from lots of other mail systems, innocent subscribers at Gmail, Hotmail, etc. not only won't get Yahoo subscribers' messages, but all those bounces are likely to bounce them off the lists.”

In other words lots of email not getting through, lots of automatic unsubscribes and lots of angry users and sysadmins.

Levine offers three suggestions to ameliorate the situation:

  • Suspend posting permission of all yahoo.com addresses, to limit damage
  • Tell Yahoo users to get a new mail account somewhere else, pronto, if they want to continue using mailing lists
  • If you know people at Yahoo, ask if perhaps this wasn't such a good idea

Who wants to be the one to action the third suggestion? ®

Internet Security Threat Report 2014

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