correct way to prevent SPF failures

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http://serverfault.com – I have a website on a hosted server whose mail users are using their ISPs SMTP to send mail. I have set their SPF record to look like this: v=spf1 mx a:comcast.net ip4: ip4: ip4: -all the SMTP host is comcast.net, is the ip range they get assigned from. ip4: ip4: are the two possible SMTP ranges they could get IF they used their hosting account mail servers [media temple] They are still getting SPF errors, any idea why? (HowTos)