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
In the comments of last week’s Wednesday question John B. asked
Read MoreLast night (Mar 29, 2017) between about 8pm Eastern and 9:30pm Eastern AOL suffered a technical issue. Every email sent to them received a “Recipient address rejected” reply. One example of the error message:
Mar 29 20:45:12 p2-lvmail11 lsb1-99-208-250/smtp[22251]: A88DFC2DBE9: to=<redacted@aol.com>, relay=mailin-01.mx.aol.com[64. 12.91.195]:25, delay=0.18, delays=0.01/0/0.14/0.03, dsn=5.1.1, status=bounced (host mailin-01.mx.aol.com[64.12.91. 195] said: 550 5.1.1 <redacted@aol.com>: Recipient address rejected: aol.com (in reply to RCPT TO command))
The issue was brought to AOLs attention and things were fixed rapidly after that. An AOL representative has stated that these were invalid replies and that addresses do not need to be removed from future emails.
Most of the ESPs are aware of this and are working to restore any bounced addresses to their users. At some places this requires manual intervention, so it’s taking some time to get all the addresses restored.
This is one of the reasons that our best bounce handling recommendations are not to remove an address for a single bounce – sometimes the ISPs have technical problems. Like the time a routing failure meant a major ISPs MX machines couldn’t reach their authentication servers to get the list of active users. Or the time all an ISPs MXs were removed from DNS. A lot of the internet is still managed manually, and despite extensive safeguards put in place bad things can, and do, still happen. Usually these problems are resolved quickly and mail starts flowing again.
Morning advice: Do not deactivate addresses that bounced at AOL last night.