Hotmail and Gmail mark emails as spam

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http://serverfault.com – I have been developing a PHP application and one of the features in that application is the ability to send e-mails to users. However, when I tried to send e-mails via application (SMTP) to Gmail and Hotmail accounts, they always go to Spam/Junk box. This doesn't happen when I use Google SMTP for sending them. Those e-mails were delivered properly to Inbox. What I have done: - Set up reverse DNS - Set up Domain Keys (DKIM) and SPF (v=spf1 +a +mx +ip4: ?all) - Run various IP/domain checks and I didn't see any problem there. - Sign up to SNDS. After checking my IP, it says "All of (HowTos)