dns
Seen this recently? 451 Message temporarily deferred due to unresolvable RFC.5321 from domain; see https://postmaster.yahooinc.com/error-codes This is Yahoo doing some extra work to identify that the 5321.From domain1 of the mail is acceptable to them. Yahoo are going (slightly) beyond what’s required for the return path to be valid in SMTP terms. SMTP just requires that the return path be syntactically valid – i.e., looks like an email address – and that it be deliverable. The basic DNS check you might do would be to check if the right-hand-side of the email address has an MX record2. So for a bounce address of bounces@email.example.com you’d check to see if email.example.com had an MX record. Yahoo want to also check that it looks like a legitimate address in another way, that the organizational domain of the right-hand-side looks legitimate. The organizational domain is what you might think of as a “domain”
A friend warned me of a scenario that could have the potential to freak people out, if misunderstood. It looks like this:This person is using Spamhaus to filter inbound mail.They seem to be rejecting mail from Gmail due to a Spamhaus listing.The Spamhaus website DOES suggest there might be an SBL entry (a blocklisting) for Gmail.So…Spamhaus is blocking Gmail? NO, no no. Gmail is not blocklisted by Spamhaus. Promise. Here’s what’s actually happening.Using Spamhaus is good, but querying Spamhaus using open/public DNS resolvers is bad. Spamhaus is actually rejecting those queries — they’re not blocking mail from Gmail. The person running into this problem needs to switch over to using the Spamhaus DQS (Data Query Service), and that ought to just flat out fix things.As noted above, the rejections are actually because the email administrator of the mailbox provider or mail server in question has configured Spamhaus in a way
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 https://register.gotowebinar.com/register/2268789893122531343
Julia Evans aka b0rk is somebody smart that I follow on Twitter. She created this super cool tool that I’ve linked to on social media before, called Mess with DNS, providing you with a safe space to learn about DNS by doing — letting you set up DNS records under a test domain she’s got set up — so you can truly “mess with DNS” without breaking anything for your own (or your employer’s) domain.And now she’s back with something new and super cool — a 28-page guide (zine) called “How DNS Works!” Guess what it explains? Yep, how DNS works. Lots and lots of detail here, very useful for somebody who wants to better understand how the domain name service actually works. And when you’ve got the guide in hand (available for a modest fee from her zines website), then head on over to “Mess with DNS” and practice…