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Tag: SMTP

January 2016: The Month in Email

Happy 2016! We started off the year with a few different “predictions” posts. As always, I don’t expect to be right about everything, but it’s a useful exercise for us to look forward and think about where things are headed. I joined nine other email experts for a Sparkpost webinar on 2016 predictions, which was […]

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Following the SMTP rules

An old blog post from 2013, that’s still relevant today. “Blocked for Bot-like Behavior” An ESP asked about this error message from Hotmail and what to do about it. “Bot-like” behaviour usually means the sending server is doing something that bots also do. It’s not always that they’re spamming, often it’s a technical issue. But […]

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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, […]

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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, […]

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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 […]

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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 […]

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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 […]

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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 […]

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  • HE.net DNS problems

    Hurricane Electric had a significant outage of their authoritative DNS servers this morning, causing them to return valid responses with no results for all(?) queries. This will have caused delivery problems for any mail going to domains using HE.net DNS - which will include some of their colocation customers, as well as users of their free services - but also will have caused reverse DNS to fail for most servers hosted by Hurricane Electric worldwide, so if any of your mail is being sent from HE hosted machines you may have seen problems. (We're HE customers so we noticed. Still happy with them as a vendor.)No Comments


  • 65.0.0.0/8 DNS issues

    If you're sending email from any address beginning with a 65 - in 65.0.0.0/8 - it's possible you'll see some delivery problems. Something appears to be broken with dnssec signatures for the reverse DNS zone, leading queries for reverse DNS to fail for anyone using a dnssec aware DNS resolver (which is almost everyone).1 Comment


  • Our green bar certificate is going away

    Later today we'll be switching from an Extended Validation ("green bar") SSL certificate to a Domain Validation certificate. This isn't exactly a planned change but I'm waiting for responses from Comodo before I go into it too much. I'll share some more details next week.3 Comments


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