How to resolve SPF configuration to allow Google sending mails?

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http://serverfault.com – we are configuring DNS for a GMail service, the MX records are configured as to Google says, however, in the DNS check, there is a warning: SPF must allow Google to send mail on behalf of the domain My questions: Why is the SPF record for GMail a problem? How to resolve that issue? Many thanks for your help! (HowTos)