The nice folks over at Postmark shared a new deliverability resource last week. The SMTP Field Manual. This is a collection of SMTP responses they’ve seen in the wild. This is a useful resource. They’re also collecting responses from other senders, meaning we can crowdsource a useful resource for email deliverability folks.
I’ve got multiple clients right now looking for insights about bounce handling. This means I’m doing a lot of thought work about bounces and what they mean and how they match up and how different ISPs manage delivery and how different ESPs manage delivery and how it all fits together. One thing I’ve been trying to do is contextualize bounces based on what the reason is.
Despite what people may thing, spam filtering isn’t the only reason an email fails to deliver. There are lots of other reasons, too. There is a whole category of network problems like routing issues, TCP failures, DNS failures and such. There are address issues where a recipient simply doesn’t exist, or is blocking a particular sender. There are spam and authentication issues. The discussion of all these issues is way longer than a blog post, and I’m working on that.
One of the interesting bounces that is so rare most people, including me, never talk about is “Relaying Denied.” This is, however, one of the easier bounces to explain.
Relaying Denied means the mail server you’re talking to does not handle mail for the domain you’re sending to.
Well, OK, but how does that happen?
There are a couple reasons you might get a “Relaying Denied” message, most of them having to do with a misconfiguration somewhere. For whatever reasons, the receiving server doesn’t handle mail for a domain.
DNS records are incorrect. These can be due to a number of things
Sometimes I find it hard to find a new topic to write about. I decide I’m going to write about X and then realize I did, often more than once. Other times I think I can blog about some issue only to realize that it’s too complex to handle in a quick post. There are concepts or issues that need background or I have to work a little harder to explain them.
One thing I haven’t blogged about before is bounce handling. That particular topic falls into the other category of posts that take a lot of time to write and need a significant amount of work to make sense. I was even joking with my fellow panel members at EEC a few months ago about how that’s a post that so needs to be written but I’m avoiding it because it’s so hard. There’s so much to be conceptualized and explained and I realize it’s not a blog post but multiple blog posts, or a white paper or even a book.
So let’s start with some simple definitions. Those of you who work at ISPs are probably thinking of bounces in terms of accept than reject, that’s not exactly what I’m talking about here. I’m writing these for senders, who usually call rejects during the SMTP transaction bounces.
In the comments of last week’s Wednesday question John B. asked
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