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 the technical problems make the sending server look like a bot, so the ISP is not taking any chances and they’re going to stop accepting mail from that server.
If you’re an ESP what should you look for when tracking down what the problem is?
First make sure your server isn’t infected with anything and that you’re not running an open relay or proxy. Second, make sure your customers aren’t compromised or have had their accounts hijacked.
Then start looking at your configuration.
HELO/EHLO values

  • are you using a consistent one for each IP or are the values changing?
  • are you using a FQDN (fully qualified domain name, i.e., a.example.com) for the HELO?
  • are you using a bare IP for the HELO/EHLO?
  • does that FQDN match the rDNS of the IP?

IP issues

  • does your IP have rDNS?
  • does that rDNS in any way look like it might be dynamic?
  • how many IPs are you using to send the same message?

Header issues

  • are you using a consistent hostname in the envelope From?
  • is that hostname similar to the hostname in the IP address?
  • are you sending a MessageID?
  • are you authenticating?
  • is that authentication correct?

Content

  • are you sending small amounts of the same content over different IP addresses?
  • is the content you’re sending being sent by other entities?
  • are any of the URLs you’re linking to infected with anything?
  • are any of the URLs you’re linking to serving ads that might be spreading viruses?

Sending issues

  • are you closing connections promptly or are you holding them open?
  • are you opening connections from different IPs and sending the same content at the same time?

The major bot-like behaviours are sending small numbers of messages from many different IP addresses, and using bad HELO/EHLO values. Even small senders using shared pools can trigger this filter at Hotmail. Try not to split small volumes of mail over multiple IPs whenever possible and particularly when you are getting this error message.
 

Related Posts

When did the reject happen?

conversation_for_blogEarlier 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:

Read More

Whirlwind that is M3AAWG

It’s been a great conference, and it’s only about half done. As is common at these conferences, I write down lots of things we should do and need to publish. The difference is now that we are growing I may have the time to put the polish on them and get them published.
Today’s keynote discussed the economics of botnet mitigation. Michel van Eeten from Delft University of Technology presented information compiled from some different datasets about botnets.
Good news
Botnet infection rates are relatively stable. They’ve not spiraled out of control like some people were predicting.
Interesting news
More than 50% of bot infections are contained on 50 ISPs in the entire world.
Bad news
Centers set up specifically to fix botnet infections don’t really have a big impact on infection cure rate.
Good news
ISP actions and walled gardens do have an impact on infection cure rates.
The biggest take away from the session is that ISPs are critical in both protecting from infection and helping users cure infection once it happens.

Read More

Increase in CBL listings

Update: As of Nov 24, 2015 11:18 Pacific, Spamhaus has rebuilt the zone and removed the broken entries. Expect the new data to propagate in 10 – 15 minutes. Delivery should be back to normal.
The CBL issued a statement, which I reposted for readers that find this post in the future. I think it’s important to remember there is a lot of malicious traffic out there and that malicious traffic affects all of us, even if we never see it.
Original Post from 10am pacific on Nov 24
cbl-logo-2012
Mid-morning west coast time, I started seeing an uptick in reports from many ESPs and marketers that they were getting listed on the XBL/CBL. Listings mentioned the kelihos spambot.

Read More