Bypass Gmail's spam filter (mails sent with PHP from a shared host)

Question

TL;DR: Mails sent from shared hosting (such as a cheap domain from Unoeuro or One.com) end up in spam. How to fix?

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I made a mail-system, that

Answer 1

There are at least two "spammy" looking things that jump out of your email headers:

Message-Id: <51450f37.6a0b700a.6239.5dbcSMTPIN_ADDED_MISSING@mx.google.com>

Notice the SMTPIN_ADDED_MISSING section? You aren't behaving like a proper mailer and generating a unique Message-ID. You might find reading RFC 5322 to be instructional.

Received: from localhost.localdomain (srv18.one.com [193.202.110.18])
by mail-out2.b-one.net (Postfix) with ESMTP id F3D0B10365
for ; Sun, 17 Mar 2013 01:32:53 +0100 (CET)

That initial received header has an illegal HELO hostname (localhost.localdomain). Your mailer app should provide a way for you to set that to a valid value. It might even be as easy as configuring the hostname of the machine running PHP. See RFC 1035 (hostname validity), RFC 2821 (SMTP) and RFC 5321 (SMTP).