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Gmail Full Headers
When it comes to spam filters of free email service providers Gmail spam filters are most sane, Gmail will only block your mails or send them to spam folder if there is pattern of spam or broken configuration from your mail server. Gmail will also provide specific error in mail marked as spam providing details as why your mail is marked as spam. Other ISP will not provide you this detail. You have to check "Why is this message in Spam" message gmail display when you open mail from spam folder.

Gmail Sending Mail to Spam Folder