Why are emails sent from my dedicated server marked as SPAM on Gmail?

Question

I have a website hosted on a dedicated server which sends automatic emails to users registered. Such emails worked until some time ago.

Today i realized those emails go straight into the SPAM folder with Gmail. A similar problem already occurred some months ago, but i fixed it as explained here . But now it doesn't work again. Opening one of those emails i see this message above it: "Marked as SPAM because It's similar to messages that were detected by our spam filters."

Here is the content of one of those emails:

Delivered-To: fontanavideostudios@gmail.com
Received: by 10.64.176.209 with SMTP id ck17csp877005iec;
        Thu, 14 Apr 2016 13:09:33 -0700 (PDT)
X-Received: by 10.194.95.167 with SMTP id dl7mr18047573wjb.163.1460664573057;
        Thu, 14 Apr 2016 13:09:33 -0700 (PDT)
Return-Path: <noreply@racebooking.net>
Received: from ns362512.ip-91-121-174.eu ([2001:41d0:1:ef28::1])
        by mx.google.com with ESMTP id 193si8350387wmo.16.2016.04.14.13.09.32
        for <fontanavideostudios@gmail.com>;
        Thu, 14 Apr 2016 13:09:32 -0700 (PDT)
Received-SPF: pass (google.com: domain of noreply@racebooking.net designates 2001:41d0:1:ef28::1 as permitted sender) client-ip=2001:41d0:1:ef28::1;
Authentication-Results: mx.google.com;
       spf=pass (google.com: domain of noreply@racebooking.net designates 2001:41d0:1:ef28::1 as permitted sender) smtp.mailfrom=noreply@racebooking.net
Received: by ns362512.ip-91-121-174.eu (Postfix, from userid 504)
    id 1925A16071; Thu, 14 Apr 2016 21:54:41 +0200 (CEST)
To: fontanavideostudios@gmail.com
Subject: =?utf-8?Q?Modifica_la_Password?=
X-PHP-Originating-Script: 504:Email.php
From: "Racebooking.net" <noreply@racebooking.net>
Reply-To: "noreply@racebooking.net" <noreply@racebooking.net>
X-Sender: noreply@racebooking.net
X-Mailer: CodeIgniter
X-Priority: 3 (Normal)
Message-ID: <570ff5810b30c@racebooking.net>
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary="B_ALT_570ff5810b35b"
Date: Thu, 14 Apr 2016 21:54:41 +0200 (CEST)

This is a multi-part message in MIME format.
Your email application may not support this format.

--B_ALT_570ff5810b35b
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit

Abbiamo ricevuto una richiesta di modifica della password.Per procedere con
la modifica, cliccare sul seguente link. Qualora non funzionasse, copia e
incolla tutto cio che vedi tra le due file di asterischi (asterischi
esclusi) e incollalo nella barra del tuo
browser:*********************************************
https://www.racebooking.net/it/moto/change_password?email=fontanavideostudios%40gmail.com&code=ff694917344f6b2c1af95ad397446942*********************************************Il
link non è valido per sempre, ma scadrà in data Thursday April 2016 alle
ore 22:54:41.Qualora, invece, non abbia richiesto lei la modifica della
password, ignori questo messaggio.


--B_ALT_570ff5810b35b
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: quoted-printable

Abbiamo ricevuto una richiesta di modifica della password.<br>Per procedere=
 con la modifica, cliccare sul seguente link. Qualora non funzionasse, copi=
a e incolla tutto cio che vedi tra le due file di asterischi (asterischi es=
clusi) e incollalo nella barra del tuo browser:<br>************************=
*********************<br> <a href=3D'https://www.racebooking.net/it/moto/ch=
ange_password?email=3Dfontanavideostudios%40gmail.com&code=3Dff694917344f6b=
2c1af95ad397446942'>https://www.racebooking.net/it/moto/change_password?ema=
il=3Dfontanavideostudios%40gmail.com&code=3Dff694917344f6b2c1af95ad39744694=
2</a><br>*********************************************<br>Il link non è va=
lido per sempre, ma scadrà in data Thursday April 2016 alle ore 22:54:41.<=
br><br>Qualora, invece, non abbia richiesto lei la modifica della password,=
 ignori questo messaggio.

--B_ALT_570ff5810b35b--

What's wrong with it?

On other providers these emails are not marked as spam, it seems to happen only with Gmail.

Answer 1

Create a line of code that sets the headers to a name of the sender has sent a message. This is what I had to do, now mine appears in the primary tab of Gmail.

$toaddress='merrimack.scioly@gmail.com';
    $subject=  $fromsub . ' ' . 'sent you a message';
    $headers = 'From: ' . $fromsub . "\r\n" .
               'Reply-To: ' . $from . "\r\n";
    $mailcontent= 'Team Member: ' . $fromsub . "\r\n"
                  .'Email: ' . $from . "\r\n"
                  .'Message: '.$message. "\r\n";
    $fromaddress= 'From: ' . $from;

    mail($toaddress, $subject, $message);