When did the reject happen?

Earlier today I approved a comment from Mike on a post about problems at AOL from 2012. The part of the comment that caught my attention: SMTP error from remote mail server after end of data: 521 5.2.1 : AOL will not accept delivery of this message. Mike also mentioned his IP reputation is good, […]


Confusing the engineers

We went camping last weekend with a bunch of friends. Had a great time relaxing on the banks of the Tuolumne River, eating way too much and visiting. On Saturday I was wearing a somewhat geeky t-shirt. It said 554: abort mission. (Thank you MessageSystems). At some point on Saturday every engineer came up to me, […]


Email is inherently a malicious traffic stream

It’s something many people don’t think about, but the majority of the traffic coming into the SMTP port is malicious. Spam is passively malicious, in that it just uses resources and bothers people. But there is a lot of actively malicious traffic coming into the SMTP port. Email is used as a vector to spread viruses […]

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The best time to send email

This subject comes up over and over again. Many senders are convinced that there is a best time to send email. Countless research hours have been dedicated to finding that best time to send email. Numerous blog posts discuss what the best time to send email is. From my perspective, there are better places for senders […]


SWAKS: the SMTP Swiss Army Knife

SWAKS is a general purpose testing tool for SMTP. For basic SMTP testing it’s a more convenient, scriptable alternative to running a transaction by hand, but it also lets you test things that are difficult to do manually, such as authentication or TLS encryption. It’s a perl script that installs fairly easily on OS X […]


SMTP Level Rejections

While discussing a draft of a Deliverability BCP document the issue came up of what rejections at different phases of the email delivery transaction can mean. That’s quite a big subject, but here’s a quick cheat sheet. At initial connection Dropped or failed connection: your reputation with the receiver is so bad that they don’t […]


Does email have a guarantee of delivery?

A client asked me earlier this week what SLAs ISPs provided for email delivery. The short answer is that there isn’t a SLA and that the only guarantee is that the email will get there when it gets there. But as I was mentioning this to Steve, he pointed out that there was a recent […]

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8 things that make your mail look like spam

In the comments of last week’s Wednesday question John B. asked Can you elaborate on specifics of “configure machines to not look like spam ware”? There are a lot of things that spamware does that is different from a lot of standard MTAs. Here are a list of things that may make your mail look […]


The Physics of the Email Universe

We talk a lot about rules and best practices in email, but we’re mostly talking about “squishy” rules-of-thumb that are based on simplified models of how mail systems, spam filters, recipients, postmasters and blacklist operators behave. They’re the biology, ecology and sociology of the email ecosystem. There’s another set of rules we tend to only […]

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Email is store and forward

Many of us are so used to email appearing instantaneous, we forget that the underlying protocol was never designed for instant messaging. When the SMTP protocol was originally proposed it was designed to support servers that may have had intermittent connectivity. The protocol allowed for email to be spooled to disk and then sent when […]

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  • Lost in the mists of time

    Over on the Farsight Security blog Joe St. Sauver talks about some of the early days of online abuse, on usenet. Laura and I were on the periphery of early usenet abuse, mostly as users, but Usenet (and IRC) around then were the places we both started with email abuse.No Comments

  • Ongoing Yahoo delays

    I've been hearing from folks over the last few days that they're seeing an uptick in deferrals from Yahoo! The deferrals are not uniform. ESPs report they're seeing some, but not all, customers affected. Other ESPs aren't seeing any changes. It's not just you. But it would be very worthwhile to dig into engagement and other stats. It's possible this is a new normal at Yahoo! and they're tightening filters to catch mail that doesn't fit their standards but was previously difficult to filter.No Comments

  • AOL starts using Sender Score Certification

    Good news for Sender Score Certified IPs. Return Path recently announced that AOL has joined the list of ISPs offering preferential treatment to certified IPs.  1 Comment