Reject mail from invalid domains

[Dhawal Doshy]

Koopmann, Jan-Peter wrote:
> On Tuesday, May 29, 2007 9:13 AM Jorge Costinha wrote:
> 
>> invalid domain, are domains that cannot be resolved by DNS. i belive
>> it is a good practice to reduce spam and workload of mailscanner and
>> spamassassin if mails coming from invalid domains are block. am i
>> right?   
> 
> In theory: Yes. The question is how do you make sure the domain can be resolved? Nameservers exist? Whois is successful (would not work probably)? Sender callout (google for SAV or Sender address verification to find out that many people seriously dislike it). And even if the domain resolves you could check if the domain has valid MX records. Or SPF and so on. 
> 
> Implementation depends on your MTA of course.

Postfix does this well.. it considers lack of A/MX record OR a malformed 
record as an invalid domain. See:
http://www.postfix.org/postconf.5.html#reject_unknown_sender_domain