How Email Spam Filters Work Based On Algorithms

This is pretty basic stuff; it's not inaccurate, but it's not complete. Most other ISPs have other things going on that can also result in emails going to the spam folder. But as a starting point? It's not too bad. Check it out, from NBC: How Email Spam Filters Work Based On Algorithms.

Need example SMTP bounces for different ISPs?

Wondering what different kind of bounces an ISP might give to you? Postmark put together this handy-dandy SMTP bounce example lookup tool. It's called the SMTP Field Manual and it's pretty neat.

Spamhaus Blacklist Changes

Speaking of Spamhaus, this just popped up in my RSS feed reader. It looks like Spamhaus is going to take a harder stance against users who query their blacklists via open or public DNS systems (such as Google Public DNS or Cloudflare's Service). They're going to respond to all queries from public/open DNS systems with a new answer code, and respond to excessive queries from other sources with a new response code. The net here is that if you query Spamhaus a lot, and aren't a registered, paying user, or if you use public DNS services for even your small hobbyist server, you're going to get cut off.

And based on the way this is implemented, it's possible that a bunch of legitimate mail will start bouncing before all Spamhaus users figure it out.

Even on my own hobbyist Linux box, I'm likely to run afoul of this. Instead of running my own DNS server, I just use Google's public DNS, and I use Spamhaus's "Zen" blacklist in my Postfix email server. Or at least I did, until I removed that DNSBL from the mail server configuration just now.

Stay tuned. I bet we're going to start seeing people popping up to ask why they're suddenly not receiving any more inbound mail.

Click here to head on over to Spamhaus to read the announcement.

Blacklists and multi-client impact: The risk is real

You hear stories sometimes. About how when a deliverability person warns sales that they shouldn't sign that client, but the client comes aboard anyway. "If they do bad things, so what? It shouldn't impact other clients. We'll give them their own domain, their own IPs, and it'll be fine."

Are you sure?

Too many times now, I've seen blacklists like SORBS or Spamhaus blacklist whole ESPs or whole large blocks of IP addresses at an ESP. I bet it's not fun explaining to client #2 that their bounce rate jumped to 50%+ because of the bad acts of client #1.

And this isn't just something that happened in the past. Just about two weeks ago I saw Spamhaus blacklist 255 IP addresses at a particular email service provider due to the actions of a single client. (The listing is since removed, so I can't link to it. And my goal isn't to name-and-shame, so I'm not mentioning which ESP it is. If you're smart, maybe you can figure it out.)

You might argue that Spamhaus appled too broad a stick and perhaps they shouldn't have done that. You might be right. Complain all you want, though, but you can't control Spamhaus, and neither can I, and neither can that ESP. But that ESP can control what clients they allow to use their services, so I would argue that they did have a way that they could have prevented this.

Assuming that one client's bad practices won't affect other clients is a risky proposition.

And I'm not even touching on what this kind of thing does to an ESP's reputation. If you want to be a member of M3AAWG, or if you want ISP people to respond to you favorably when you sometimes ask for help out of band, you need to not have the reputation that your platform will take any client, even ones with bad practices.

Let's go buy a list!

Is buying an email list a good idea? Let's ask around.

Hubspot's got the best quick summary, in my opinion. They say buying an email is a bad idea because:
  1. Reputable email marketing services don't let you send emails to lists you've bought.
  2. Good email address lists aren't for sale.
  3. People on a purchased or rented list don't actually know you.
  4. You'll harm your email deliverability and IP reputation.
  5. You can come across as annoying.
  6. Your email service provider can penalize you.
Don't believe them? Ask Campaign Monitor.
Don't believe Campaign Monitor? Ask Constant Contact.
Don't believe Constant Contact? Ask Godaddy.
Don't believe Godaddy? Ask HostGator.
Don't believe HostGator? Ask SparkPost.
Don't believe SparkPost? Ask GetResponse.
Don't believe GetResponse? Ask Vertical Response.
Don't believe Vertical Response? Ask WhatCounts.

I could keep going...but you get the idea.

HOWTO: Work around Office 365 Unblocking Issues

If you've previously seen this bounce message:
5.7.606 Access denied, banned sending IP []. To request removal from this list please visit and follow the directions. For more information please go to []"