legitimate mails marked as SPAM: find solution (sender ID,PTR, DNS, Google SMTP, etc)

I? have somebody from rentacoder who has developed a new classified ads program. The programming is reaching its final stage. However, some of the emails that the program sends are being marked as SPAM. For example mails to hotmail servers don't reach their destination properly. Also Gmail sometimes gives problems. It's your job to tell me what changes I should make to the server, to my website code, or suggest other solutions (Google SMTP?) and explain everything in detail. (Sender ID, PTR, Reverse DNS, signing and cetere)? I've checked Blacklists but couldn't find my [login to view URL] programming is done in PHP.? My server is a Linux VPS with Plesk installed.? In the attached document i have the email headers for mails that arrive in SPAM. You van also see the DNS settings for my server.? I can also provide you with the mail logs from the server. You should talk me through all changes So your instructions should be crystal clear.