PHP Mail() to Gmail = Spam

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http://serverfault.com – Recently Gmail has started marking emails sent directly from my server (using php mail()) as spam and I'm having problems trying to find the issue. If I send an exact copy of the same email from my email client it goes to the Gmail inbox. The emails are plain text, around 7 lines long and contain a URL link in plain text. As the emails sent from my client are getting through fine I'm thinking that the content isn't the issue. It would be greatly appreciated if someone could take a look at the the following headers and give me some advice why the email from the server is being marked as spam. (HowTos)