spf records

http://www.openspf.org/FAQ/Common_mistakes
http://www.kitterman.com/spf/validate.html

example.com.             IN  TXT  "v=spf1 mx -all"


electronics.com 1   TXT v=spf1 a:backup.co.uk a:mail.a.co.uk a:smtp.id.net a:smtp-out.id.net a:mail.it.com ~all


Publish null SPF records for your domains that don't send mail

Once you've protected your mail sending domains with SPF, if someone is trying to spoof you, then first thing they will try is to spoof your non-mail sending domains. Publishing "v=spf1 -all" says that a domain sends no mail. As an example, you might publish:

    example.com.       IN  TXT  "v=spf1 a:mail.example.com -all"
    mail.example.com.  IN  TXT  "v=spf1 a -all"
    www.example.com.   IN  TXT  "v=spf1 -all"