When trying to find out why Something Went Wrong during delivery of an email we sometimes want to look at the route by which it was delivered. Did SPF break because of an unexpected forward? Did DKIM break because an intermediate mailserver modified the content of the message? Why did it take nine hours from the mail leaving our ESP to make it to the inbox? Did it really leave our ESP when they...
TXT Records DKIM public keys live in DNS TXT records. A DNS TXT record contains strings of text, and each string is limited to be no more than 255 characters long. Recommended practice for DKIM at the moment is to use 2048 bit keys (1024 bit keys aren’t insecure, but they’re looking a bit weak and 2048 is where folks have mostly decided to move to). But a 2048 bit DKIM key is going to...
The worst thing about the yahoogle requirements has been their use of the term “one-click unsubscribe”. It’s an overloaded term that’s being used here to mean RFC 8058 in-app unsubscription. That’s a completely different thing to what one-click unsubscription has been used to mean for decades, often in the context of complying with legal requirements around...
It’s not always easy to know what the actual headers and body of an email as sent look like. For a long time accepted wisdom was that you could send a copy to your gmail account, and use the Show Original menu option to, well, see the original message as raw text. It turns out that’s not actually something you can trust. I used swaks to send a test message with an extra header to my...
Happy 2024, everyone! We’ve released a shiny new tool to let folks self-check a lot of common questions we see about email requirements. Go to AboutMy.email and send an email to the email address it gives you. Once it receives that email it will go through it and do many of the basic checks we’d usually do to check the technical health of a client’s email1AboutMy.email is a...
Email supports TLS (Transport Layer Security), what we used to call SSL. Unlike the web, which split it’s TLS support off into a completely different protocol – https, listening on port 443 vs http listening on port 80 – SMTP implements it inside it’s non-encrypted protocol. A mailserver advertises that it supports this by having the word “STARTTLS” in the...
EDIT: Now with a production-ready implementation I talk about more here. On Tuesday I wrote about using DNS wildcards to implement customer-specific subdomains for email authentication. As I said then, that approach isn’t perfect. You’d much prefer to have per-customer domain authentication, where each customer has their own DKIM d= and ideally their own SPF records, rather than...
If you’re an ESP with small customers you may have looked at the recent Google / Yahoo requirements around DMARC-style alignment for authentication and panicked a bit. Don’t impersonate Gmail From: headers. Gmail will begin using a DMARC quarantine enforcement policy, and impersonating Gmail From: headers might impact your email delivery.…For direct mail, the domain in the...
When you query DNS for something you ask your local DNS recursive resolver for all answers it has about a hostname of a certain type. If you’re going to a website your browser asks your resolver for all records for “google.com” of type “A”1or “AAAA”, but that’s not important right now and it will either return all the A records for google.com it has...
Eventually our subscribers won’t want our email in their inbox any more. They can stop the mail either by unsubscribing from it, or by marking it as spam. We’d far rather they do the first so we should make it as easy as possible for them to unsubscribe. Also in most jurisdictions you’re legally required to offer a functional, easy to use unsubscription channel. So, how to do...
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