Subject tagging and DMARC

Pobox email forwarding works by passing messages through with no modifications -- except for email filters with subject tagging. To date, this has been a simple and effective way of indicating different kinds of information in an obvious place.

In April 2014, Yahoo made a change in their email policy that could affect messages that use subject tags. They use DMARC, which is an authentication and spam-filtering protocol meant to be used for transactional messages. They told ISPs who use DMARC to begin bouncing mail sent from Yahoo users that does not authenticate. 

DMARC uses 2 existing authentication standards, SPF and DKIM, to determine whether or not to take action on a message. DKIM uses a signature, generated using elements of the message including Subject, to make sure the message hasn't been altered since it was signed. Changing your email message subject breaks DKIM. If you receive mail from a Yahoo user, and use subject tagging to change the subject, many major ISPs, including Gmail, Yahoo, Comcast, Outlook (formerly MSN and Hotmail), and more, will bounce the message.

Pobox filters already include a header you can use for filtering in your email client. Messages that match one of your filters will include header:

X-Pobox-Filter-Match: <rule matched>


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