C # email for gmail marked spam, but thunderbird client comes as usual (not spam)

I have a small vps where mailenable is installed. I already checked the PTR, changed the DNS and server settings, everything is in order.

If I send an email from Thunderbird to gmail, everything is fine and the mail is not marked as spam, this is the header:

Delivered-To: antani2@gmail.com
Received: by 10.60.38.104 with SMTP id f8csp54812oek;
Fri, 18 May 2012 13:04:27 -0700 (PDT)
Received: by 10.216.138.130 with SMTP id a2mr7849178wej.35.1337371466927;
Fri, 18 May 2012 13:04:26 -0700 (PDT)
Return-Path: <mailrobot@charterino.com>
Received: from mail.charterino.com (mail.charterino.com. [95.110.146.72])
by mx.google.com with ESMTP id m60si12747577wee.64.2012.05.18.13.04.26;
Fri, 18 May 2012 13:04:26 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) client-ip=95.110.146.72;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) smtp.mail=mailrobot@charterino.com
Received: from [192.168.1.132] ([2.227.153.144]) by charterino.com with MailEnable ESMTP; Fri, 18 May 2012 22:01:40 +0200
Message-ID: <4FB6AB4A.8000508@charterino.com>
Date: Fri, 18 May 2012 22:04:26 +0200
From: "mailrobot@charterino.com" <mailrobot@charterino.com>
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20120428 Thunderbird/12.0.1
MIME-Version: 1.0
To: antani2@gmail.com
Subject: Prova
Content-Type: text/plain; charset=ISO-8859-15; format=flowed
Content-Transfer-Encoding: 7bit

When I send an email from a C # application, my message is marked as spam, this is the header:

Delivered-To: antani@gmail.com
Received: by 10.229.39.205 with SMTP id h13csp119310qce;
Fri, 18 May 2012 12:09:26 -0700 (PDT)
Received: by 10.180.82.136 with SMTP id i8mr4157077wiy.19.1337368166529;
Fri, 18 May 2012 12:09:26 -0700 (PDT)
Return-Path: <mailrobot@charterino.com>
Received: from mail.charterino.com (mail.charterino.com. [95.110.146.72])
by mx.google.com with ESMTP id h11si2000858wiw.14.2012.05.18.12.09.26;
Fri, 18 May 2012 12:09:26 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) client-ip=95.110.146.72;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) smtp.mail=mailrobot@charterino.com
Message-Id: <4fb69e66.2b4cb40a.29ee.ffffa974SMTPIN_ADDED@mx.google.com>
Received: from win18864 ([::1]) by charterino.com with MailEnable ESMTP; Fri, 18 May 2012 21:06:44 +0200
MIME-Version: 1.0
From: mailrobot@charterino.com
To: antani@gmail.com

Could this be caused by a message that does not contain the @ charterino.com domain? I also tried to override the message id in the message headers from my application, but the server overwrites it.

message.Headers.Add("Message-Id","<values>@charterino.com");

I do not know if this is a problem, but ... why can thunderbird overwrite the message identifier, and I can not?

I am using System.Net.Mail.

?

"" "" ,

message.Headers.Add(String.format("<date format>",DateTime.Now.ToString()));

Delivered-To: antani@gmail.com
Received: by 10.229.39.205 with SMTP id h13csp155984qce;
        Sat, 19 May 2012 06:20:49 -0700 (PDT)
Received: by 10.216.138.130 with SMTP id a2mr9265313wej.35.1337433648907;
        Sat, 19 May 2012 06:20:48 -0700 (PDT)
Return-Path: <mailrobot@charterino.com>
Received: from mail.charterino.com (mail.charterino.com. [95.110.146.72])
        by mx.google.com with ESMTP id p8si14856169weh.0.2012.05.19.06.20.48;
        Sat, 19 May 2012 06:20:48 -0700 (PDT)
Received-SPF: pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) client-ip=95.110.146.72;
Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of mailrobot@charterino.com designates 95.110.146.72 as permitted sender) smtp.mail=mailrobot@charterino.com
Message-Id: <4fb79e30.8878d80a.4344.ffff961dSMTPIN_ADDED@mx.google.com>
Received: from win18864 ([::1]) by charterino.com with MailEnable ESMTP; Sat, 19 May 2012 15:18:08 +0200
Subject: Invio password smarrita Charterino.com
MIME-Version: 1.0
From: mailrobot@charterino.com
To: antani@gmail.com
Date: 19 May 2012 15:18:08 +0200
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: base64

, .