Block Mail from Unresolvable Domains
We have previously established that most Spam is sent with a forged envelope sender address. It turns out that spammers aren't too bright - many of the domain names they use in these addresses are not resolvable using DNS. Since the E-mail RFCs clearly state that an E-mail address is not valid if the domain name given is not DNS resolvable, it is acceptable to refuse E-mail from domains that do not resolve.
Most modern E-mail server software includes an option to return a temporary SMTP failure to delivery attempts from unresolvable domain names. A temporary failure is necessary to prevent legitimate E-mail from bouncing due to a temporary DNS connectivity issue. The sending server will retry for several days, at least, to get the mail through. Spam with a bad domain name will eventually fail on the sending server without ever coming through to your system.
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