Chip Marshall:
> Came across something odd today. I was trying to sign up at fuelly.com,
> but my Postfix mail server is rejecting their signup confirmation e-mail
> with the following:
>
> Aug 8 12:03:47 kyzoku postfix/smtpd[38670]: NOQUEUE: reject: RCPT from
> fuelly.com[174.133.216.50]: 450 4.1.8 <[EMAIL PROTECTED]>: Sender address
> rejected: Malformed DNS server reply; from=<[EMAIL PROTECTED]> to=<[EMAIL
> PROTECTED]> proto=ESMTP helo=<fuelly.com>
>
Postfix also logs a warning.
> I decided to do some digging into their DNS, and found an odd-looking MX
> record:
>
> $ dig fuelly.com mx +short
> 5 .
>
> I've never seen the root used as a mail exchanger before, and am
> guessing this is seen as a bad answer. At the very least, . has no A or
> AAAA records.
>
> Is this some sort of odd anti-spam thing? Or is the domain just set up
> badly?
It is an anti-spam hack. By my reading of the RFCs, a zero-length
MX hostname is not a valid domain.
Postfix complains about the malformed reply, and does not attempt
to look up the A record, because it did not get a correctly formed
MX response. So the hack works.
Wietse
