I wrote last year about using “stunt” nameservers for customer subdomain authentication – i.e. dynamically generating all the authentication records needed in DNS for each customer as needed. For example, if you’re an ESP that has customers who can’t or won’t use their own domains and you still need to give them unique subdomains you can generate CNAME records...
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...
Since I wrote about it last month the requirements for bulk senders to Yahoo and Google have changed a little.
The big change is that bulk senders need to authenticate with both SPF and DKIM, rather than SPF or DKIM. Only one of those has to align with the 822 From: header.
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...
One of the most common refrains I hear from folks with delivery problems is that the filters must have changed because their mail suddenly started to go to the bulk folder. A few years ago, I posted about how even when there is no change in the sender’s behavior, reputation can slowly erode until mail suddenly goes to the Gmail bulk folder. Much of that still applies – although the...
Seen this recently? 451 Message temporarily deferred due to unresolvable RFC.5321 from domain; see This is Yahoo doing some extra work to identify that the 5321.From domain1The return-path, aka the 821.From, 5321.From, or bounce address is the email address you send from at the protocol level, not the email address in the From: header, and it’s the address any bounces will be sent to. of...
When we’re looking at the technical details of email addresses there are two quite different contexts we talk about. One is an “821 address” or “5321 address”. This is the email address as it’s used by the SMTP protocol, as part of the “MAIL FROM: <>” or “RCPT TO: <>” commands sent to the mailserver. It’s defined in RFC 821...
It looks like Microsoft are getting pickier about email address syntax, rejecting mail that uses illegal address formats. That might be what’s causing that “550 5.6.0 CAT.InvalidContent.Exception: DataSourceOperationException, proxyAddress: prefix not supported – ; cannot handle content of message” rejection. Why do we care? It’s good to send syntactically valid...
I’m repeating the presentation I gave at M3AAWG in London for the Certified Senders Alliance.It’s all about how to send an email by hand, and how knowing the mechanics of how an email is sent can help us diagnose email delivery issues.We’re starting in about five hours from when I post this.Register at
I did a class at M3AAWG teaching the basic mechanics of sending an email, both really by hand using dig and netcat, and using SWAKS. No slides, but if you’re interested in the script I’ve posted a very rough copy of my working notes here.
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