Weird spammer behavior: a non-relaying relay attempt
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 hisokusa.cs.toronto.edu go-smtpd HELO smelektronik.de 250 hisokusa.cs.toronto.edu Hello 184.108.40.206 MAIL FROM: <email@example.com> 250 Okay, I'll believe you for now RCPT TO: <XXXX@hawkwind.utcs.utoronto.ca> 250 Okay, I'll believe you for now RCPT TO: <firstname.lastname@example.org> 250 Okay, I'll believe you for now RCPT TO: <email@example.com> 250 Okay, I'll believe you for now DATA ....
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
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.)