Authentication Cheat Sheet

There are a several approaches to authenticating email, and the different authentication methods have a lot of different settings to choose from (sometimes because they’re useful, other times just because they were designed by committee). It’s nice that they have that flexibility for the complex situations that might benefit from them, but almost all the time you just want to choose a good, default authentication approach.
So here’s some short prescriptive advice in no particular order for “how to do email authentication at an ESP well” without the long discussions of alternative approaches and justification of each piece of advice.

  1. Remove every trace of DomainKeys from your email flow
  2. Sign all your email with DKIM
  3. Publish SPF records
  4. Have your SPF records finish with ~all, not -all
  5. Ignore SenderID, unless you have delivery problems at Hotmail (and even then measure results rather than just assuming it will help)
  6. Don’t publish ADSP records
  7. Have the customer pick a single domain to tie their reputation to – ideally their “main” domain, but if that’s not possible then a customer-specific domain owned by the ESP is the next best thing (i.e. “customer.com” is ideal, but “customer.esp.com” is much better than just “esp.com”)
    1. Use that domain as the d= field when signing email (“d=customer.com”)
    2. Use that domain, or a sub-domain of it, in the From: field of the email (“From: Someone <someone@customer.com>” or “From: Someone <someone@fooble.customer.com>”)
  8. Pick a good email address to use in the From: field, don’t change it lightly
  9. Use two-part selectors for DKIM, one specific to you, the ESP, on the right and one for key rotation on the left (why do this?)
  10. Use just what’s needed for DKIM – don’t use i=, l=, q=, x= or z= in the signature, don’t use g= or s= in the published key
    1. dkimcore.org has more specific advice about which subset of DKIM to sign with
  11. Make sure your SPF records are valid
  12. TLS/SSL isn’t useful for authenticating mail (it’s great for connecting to a smarthost from a mail client, but not for connections from the smarthost)
  13. None of SPF, DKIM or TLS are really designed to protect or hide the contents of a message
    1. Customers who really want that could look at S/MIME or PGP
    2. … but better to design their business model such that they don’t need to, as client support is spotty at best
  14. Empirical evidence that doing something will make a customer happy trumps any of the above
    1. maybe because there’s evidence SenderID or ADSP or somesuch really helps that customer
    2. or maybe because the customer just wants it, even if there’s no evidence it would help

Mail client use of authentication data is still in flux, so it’s likely that some of this may change slightly (interaction between SPF and DKIM d= domains, for example) but it’s a pretty good base to start with today.

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The real situation is, unsurprisingly, a bit more complicated.
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Yesterday Gmail rolled out some changes to their interface. One of the changes is that they are now showing end users authentication results in the user screen.
It’s really the next step in email authentication, showing the results to the end user.
So how does Google do this? Google is checking both SPF and DKIM. If mail is authenticated and the authentication matches the from address then they display the email as:
mail from steve to me
If we click on “details” for that message, we find more specific information.
full details of message showing signing domain and spf domainIn this case the mail went through our outgoing mailserver to gmail.
Mailed-by indicates that the message passed SPF and that the IP address is a valid source of mail from wordtothewise.com.
Signed-by shows the domain in the DKIM d=. In this case, we signed with the subdomain dt.wordtothewise.com. That’s what happens when you sign using the domain in the From address (or a subdomain of it).
For a lot of bulk senders, though, their mail is signed using their ESP’s domain instead.  In that case Gmail shows who signed the mail as well as the from address.

And when we click on “details” for that message we see:
3rd party signature detailsThis is an email from a sender using Madmimi as an ESP. Madmimi is handling both the SPF authentication and the DKIM authentication.
As an aside, this particular  sender has a high enough reputation that Gmail is offering me an unsubscribe option in their interface.
Gmail is distinguishing between first party and third party signatures in authentication. If the mail is authenticated, but the authentication appears to be handled by a separate entity, then Gmail is alerting recipients to that fact.
What does this mean for bulk senders?
For senders that are signing with a domain that matches their From: domain, there is no change. Recipients will not see any mention of your ESP in the headers.
However, if you are using an ESP that is signing your mail with a domain they own, then your recipients will see that information displayed in the email interface. If you don’t want this to be displayed by Gmail, then you will need to move to first party signing. Talk to your ESP about this. If they’re unsure of how to manage it, you can point them to DKIM Core for an Email Service Provider.
Gmail blogpost about the changes
Gmail help page about authentication results

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