SenderID is dead

A question came up on the email geeks slack channel (Join Here) about SenderID. They recently had a customer ask for SenderID authentication.

We’ve written about it a few times: (Hotmail moves to SPF Authentication and Until it stops moving) but we’ve not actually stated the reasons why in a post.

SenderID was basically SPF version 2. It tried to use the same mechanism as SPF to authenticate the visible from address. In some ways it was a predecessor to DMARC. It was an authentication method championed by Microsoft.

The really important thing to remember is that Hotmail was the only domain that really used SenderID. It was a check built into some versions of Exchange servers, too. But it was never really used outside of Microsoft.

In 2012, the IETF published an informational RFC that looked at deployment of SPF and Sender ID. The author looked at a number of different things and concluded there wasn’t much use of Sender ID.

The absence of significant adoption of the [SUBMITTER] extension, SENDER-ID], and [PRA], indicates that there is not a strong community deploying and using these protocols.

Six months after that RFC was published, Microsoft announced they were moving away from Sender ID. Given they were the only major implementation, this was the signal that it was a dead authentication method.

There is no reason to publish Sender ID records. It’s dead.

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Are they using DKIM?

It’s easy to tell if a domain is using SPF – look up the TXT record for the domain and see if any of them begin with “v=spf1”. If one does, they’re using SPF. If none do, they’re not. (If more than one does? They’re publishing invalid SPF.)
AOL are publishing SPF. Geocities aren’t.
For DKIM it’s harder, as a DKIM key isn’t published at a well-known place in DNS. Instead, each signed email includes a “selector” and you look up a record by combining that selector with the fixed string “._domainkey.” and the domain.
If you have DKIM-signed mail from them then you can find the selector (s=) in the DKIM-Signature header and look up the key. For example, Amazon are using a selector of “taugkdi5ljtmsua4uibbmo5mda3r2q3v”, so I can look up TXT records for “taugkdi5ljtmsua4uibbmo5mda3r2q3v._domainkey.amazon.com“, see that there’s a TXT record returned and know there’s a DKIM key.
That’s a particularly obscure selector, probably one they’re using to track DKIM lookups to the user the mail was sent to, but even if a company is using a selector like “jun2016” you’re unlikely to be able to guess it.
But there’s a detail in the DNS spec that says that if a hostname exists, meaning it’s in DNS, then all the hostnames “above” it in the DNS tree also exist (even if there are no DNS records for them). So if anything,_domainkey.example.com exists in DNS, so does _domainkey.example.com. And, conversely, if _domainkey.example.com doesn’t exist, no subdomain of it exists either.
What does it mean for a hostname to exist in DNS? That’s defined by the two most common responses you get to a DNS query.
One is “NOERROR” – it means that the hostname you asked about exists, even if there are no resource records returned for the particular record type you asked about.
The other is “NXDOMAIN” – it means that the hostname you asked about doesn’t exist, for any record type.
So if you look up _domainkey.aol.com you’ll see a “NOERROR” response, and know that AOL have published DKIM public keys and so are probably using DKIM.
(This is where Steve tries to find a domain that isn’t publishing DKIM keys … Ah! Al’s blog!)
If you look up _domainkey.spamresource.com you’ll see an “NXDOMAIN” response, so you know Al isn’t publishing any DKIM public keys, so isn’t sending any DKIM signed mail using that domain.
This isn’t 100% reliable, unfortunately. Some nameservers will (wrongly) return an NXDOMAIN even if there are subdomains, so you might sometimes get an NXDOMAIN even for a domain that is publishing DKIM. shrug
Sometimes you’ll see an actual TXT record in response – e.g. Yahoo or EBay – that’s detritus left over from the days of DomainKeys, a DomainKeys policy record, and it means nothing today.

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SPF ?all

The most read post on the blog is Authenticating with SPF: -all or ~all. In fact, it’s in the top 5 posts every single day. We still get comments on it, too. Usually from folks who disagree with my recommendations.
I still stand by my recommendations, though. It doesn’t really matter if you choose ~all or -all in your SPF records. Why? No major provider is rejecting mail solely because of a SPF fail. They may bulk the mail, but they won’t reject it. That’s why, in a deliverability context, it doesn’t matter which one you choose.
My one rule for SPF is never use ?all. Just. No. In the spec, ?all is “testing” mode. But it really is a signifier that the person who put the SPF record together doesn’t know what they’re doing. Unless they really are testing, but even then you shouldn’t see ?all on records for weeks or months.
~ or – never ?

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Fun with opinions

Over the last few weeks I’ve seen a couple people get on mailing lists and make pronouncements about email. It’s great to have opinions and it’s great to share them. But they’re always a little bit right… and a little bit wrong.

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