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Category: Technical

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 […]

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Protocol-relative URLs in email

When you link to an external resource – an image, a javascript file, some css style – from a web page you do so with a URL, usually something like “https://example.com/blahblah.css” or “http://example.com/blahblah.css”. The world is beginning to go all https, all the time, but until recently good practice was to make a web page available […]

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ARC: Authenticated Received Chain

On Friday I talked a little about DMARC being a negative assertion rather than an authentication method, and also about how and when it could be deployed without causing problems. Today, how DMARC went wrong and a partial fix for it that is coming down the standards pipeline. What breaks? DMARC (with p=reject) risks causing problems any […]

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The philosophy of DMARC

We know that legitimate email sent with valid SPF and a DKIM signature often breaks in transit. SPF will fail any time mail is forwarded – via a mailing list, a forwarding service used by the recipient, or just ad-hoc forwarding. DKIM will fail any time the message is modified in transit. That can be obviously […]

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Tools!

I just added a DMARC validation tool over on tools.wordtothewise.com. You can give it a domain – such as ebay.com – and it will fetch the DMARC record, then explain and validate it. Or you can paste the DMARC record you’re planning to publish into it,  to validate it before you go live. If you’ve not […]

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The twilight of /8s

A “/8” is a block of 16,777,214 usable IP addresses. That’s a big fraction of the entire IPv4 address space – about 1/224, in fact. Each one is all the addresses that begin with a given number: 10.0.0.0/8 is all the IP addresses that begin with “10.”, “184.0.0.0/8” (or “184/8” for short) is all the […]

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A due diligence story

due diligence noun. research and analysis of a company or organization done in preparation for a business transaction It’s a term that’s been around for five centuries or so. Originally it meant the effort that was necessary for something, but it evolved into a legal term for “the care that a reasonable person takes to avoid […]

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Why so many IP addresses?

Hi Laura, Merry Xmas and wishing you a Happy New Year! I recently looked at a popular ESP’s IPv4 space and I was astounded. How does an ESP get an IP allocation of 20,480 IPs? ARIN guidelines do not allow “MX/Mailing” IPs to count towards a valid justification especially in the case when each and […]

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Is your website up? Are you sure?

“What would you do for 25% more sales?” It’s panicked gift-buying season, and I got mail this morning from Boutique Academia, part of their final push before Christmas. They’re hoping for some Christmas sales in the next three days. They do make some lovely jewelry – ask Laura about her necklace some time – so […]

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DKIM Canonicalization – or – why Microsoft breaks your mail

One of these things is just like the other Canonicalization is about comparing things to see if they’re the same. Sometimes you want to do a “fuzzy” comparison, to see if two things are interchangeable for your purposes, even if they’re not exactly identical. As a concrete example, these two email addresses: (Steve) steve@wordtothewise.com “Also Steve” […]

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