Asked a similar question a few years ago, ended up switching to Mailgun out of frustration. However, after a while I did get PHP mail to work with Postfix, however since then I have moved to a new domain, and all emails are once again going to spam. Here is the header for an email I just attempted to send:

Delivered-To: REMOVED
Received: by 10.28.61.9 with SMTP id k9csp2893351wma;
        Wed, 18 May 2016 09:46:21 -0700 (PDT)
X-Received: by 10.194.143.66 with SMTP id sc2mr8618309wjb.105.1463589981978;
        Wed, 18 May 2016 09:46:21 -0700 (PDT)
Return-Path: <lostusername@tptennis.net>
Received: from tptennis.net (tptennis.net. [178.62.0.187])
        by mx.google.com with ESMTP id wa2si11436041wjc.62.2016.05.18.09.46.21
        for <REMOVED>;
        Wed, 18 May 2016 09:46:21 -0700 (PDT)
Received-SPF: pass (google.com: domain of lostusername@tptennis.net designates 178.62.0.187 as permitted sender) client-ip=178.62.0.187;
Authentication-Results: mx.google.com;
       dkim=pass header.i=@tptennis.net;
       spf=pass (google.com: domain of lostusername@tptennis.net designates 178.62.0.187 as permitted sender) smtp.mailfrom=lostusername@tptennis.net;
       dmarc=pass (p=NONE dis=NONE) header.from=tptennis.net
Received: by tptennis.net (Postfix, from userid 33)
    id CCA8A440C9; Wed, 18 May 2016 16:46:20 +0000 (GMT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=tptennis.net; s=mail;
    t=1463589980; bh=HZn9uvYSPY3pIaPJQU/WXOraQOjFGWTOhME4zzh4IFc=;
    h=To:Subject:From:Reply-To:Date:From;
    b=ZaTGT3mnU5SCD/LAtBAt3c4fMdVjTd0DZrpFwicLnrQKnKW1UutAEt2Rk8fO51MfR
     KQoSPRrPMpuoSUdZggXc28h1pzOMYjDCtelTOUqMkflf/K65N15aOZXgY6J4xADIOH
     qHeqiACOvzrRonOPxzEPIGSiYeNVTOiu+DQZlGpo=
To: REMOVED
Subject: Lost username
From: TP Tennis <lostusername@tptennis.net>
Reply-To: contact@tptennis.net
Organization: TP Tennis
Message-Id: <20160518164620.CCA8A440C9@tptennis.net>
Date: Wed, 18 May 2016 16:46:20 +0000 (GMT)

You requested to have your username at TP Tennis sent back to you.

Your username is: username

Any help is appreciated