Quoth Bob Cox at 2009-03-18 18:39... ...
The question is whether you should be rejecting email from any user@act.gov.au just because act.gov.au does not resolve.
...Tempting though it is, rejecting mail on the basis of RFC-non compliance is NOT a good move. E-mail is borked beyond redemption so, until we are redeemed by the secure, efficient, replacement for SMTP, we need to be tolerant of such things as MX records not having corresponding A records - even though it makes our spam-filtering configuration that much harder.
Like Bob, I'm a Postfix man so can't really give you any Sendmail anti-spam pointers other than Spamhaus' Zen DNSBL. (I'm assuming that you are getting your issues due to anti-spam configuration stuff.)
Cheers M -- Matthew Smith Smiffytech - Technology Consulting & Web Application Development Business: http://www.smiffytech.com/ Blog/personal: http://www.smiffysplace.com/ LinkedIn: http://www.linkedin.com/in/smiffy Skype: msmiffy Twitter: @smiffytech