Weird spammer behavior: a non-relaying relay attempt

June 15, 2014

One of the the interesting things about running a sinkhole SMTP server that accepts everything and basically serves as a spamtrap is that I get to see all sorts of odd and crazy spammer behavior. Take the following SMTP transaction log:

220 go-smtpd
250 Hello
250 Okay, I'll believe you for now
250 Okay, I'll believe you for now
250 Okay, I'll believe you for now
250 Okay, I'll believe you for now

This is a CBL-listed IP address and the spaces after the ':' in MAIL FROM and RCPT TO is typical of badly implemented spamware (it's not RFC-compliant, although many mailers will accept it). The interesting thing is the second and third RCPT TO addresses. My sinkhole here is not the MX target for any of them (of course).

Sometimes you'll see deliberate relay attempt probes, but this doesn't seem to be one of them. Instead it looks like the spammer's software is just clumping lexically similar domains together and then dumping N addresses one the MX target of the first one, regardless of whether the additional addresses will ever get accepted (almost no MTA will, because almost all are configured to not relay these days).

About all I can guess is that someone wrote software that either has a bug or that is simply extremely sloppy and wrong, and the authors either never tested it or don't care. Perhaps they make their money from selling it to people who simply don't notice that an appreciable amount of their delivery attempts can never succeed. I suppose the customers are probably not in a position to really notice this behavior.

(My logs shows three such attempts so far in a few days, from two different IPs in total. It all appears to be the same spam run.)

Written on 15 June 2014.
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Last modified: Sun Jun 15 00:09:54 2014
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