Tony Finch – blog

I wanted to compile some statistics on the correctness of SMTP clients HELO domains. This is exclusively for emal coming into our MXs, so doesn’t include MUAs which tend to be very broken in this respect.

Exim in our configuration checks that the HELO domain and the reverse DNS and the forward DNS all match. However I’m also interested in whether a forward lookup on the HELO domain matches the client’s IP address, and Exim doesn’t record this in the logs. A quick bit of hackery with adns, and a few minutes of 10,000 concurrent DNS queries later, I have my results:

Total rejections: 123921 Failed HELO checks: 101417 Forward DNS correct: 2128

Total accepted: 31754 Failed HELO checks: 13349 Forward DNS correct: 3196

So, today this machine has rejected 80% of incoming messages. According to SpamAssassin about 15% of the messages we accept are spam so you might want to adjust the numbers on that basis.

Of the rejected messages, 80% have a completely bad HELO domain, and 2% have a HELO domain that’s correct only in the forward direction.

Of the accepted messages, 32% have a completely bad HELO domain, and 10% have a HELO domain that’s correct only in the forward direction.

I really like adns :-)