false positive

Hi, I have a little problem: I can't reach my user by email.
I have a service on my website in which students make questions using
a web form.
They are registered users, so when the tutor answers the questions, an
email is sent to the student mailbox.

Until now, I've used php mail function to send e-mail to students.
But with Php mail, I'm not able to know if the mail reachs the user.
So I'm trying to use Phpmailer, a software able to work as an SMTP
server.
In this software (probably all of you knows this software), you can
choose if you want to send email using sendmail, or using mail function
or using smtp server commands.

During the tests, I've tried to send an email by smtp and by mail
function. The email sent by smtp was put in junk folder in my hotmail
inbox, while the email sent with mail function got correctly in my
inbox.

I've tried to search and to understand the reasons for this failure,
but I'm not sure, and I don't know how to resolve this problem.
I send a monthly newsletter to my users, and if I use smtp, I'm able
to delete the old email accounts by using smtp error code returned by
contacted server.
But if I use mail function, this is not possible anymore.

Here are the two headers of my mail. The first email reached the inbox,
the second reached the junk folder.

Help me please!

-----------------------------------------------------------
X-Message-Status: n
X-SID-PRA: Tutor On Line
X-SID-Result: TempError
X-Message-Info: JGTYoYF78jFU0Xp676ySip6N9PNjXZZVMG5FIMxMTJo=
Received: from localhost.localdomain ([SERVERIPADDRESS]) by
mc2-f15.hotmail.com with Microsoft SMTPSVC(6.0.3790.211);
Thu, 23 Jun 2005 03:28:40 -0700
Received: (from nobody@localhost)
by localhost.localdomain (8.11.6/8.11.6) id j5NAWWJ17012;
Thu, 23 Jun 2005 12:32:32 +0200
To: mypersonaladdress@hotmail.com
Subject: Risposta del Tutor on Line
Date: Thu, 23 Jun 2005 12:32:32 +0200
From: Tutor On Line
Message-ID:
<2fc2e22c3a3224b66222aa1df8507c13@tutoring.anothermycompanyaddress.it>
X-Priority: 3
X-Mailer: PHPMailer [version 1.72]
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="iso-8859-1"
Return-Path: nobody@localhost.localdomain
X-OriginalArrivalTime: 23 Jun 2005 10:28:40.0829 (UTC)
FILETIME=[52F6FAD0:01C577DE]
-----------------------------------------------------------

X-Message-Status: n
X-SID-PRA: Tutor On Line
X-SID-Result: TempError
X-Message-Info: 6sSXyD95QpXnQGZsrRj9SJY67ZmZgN9lzzmJwNoSakc=
Received: from tutoring.anothermycompanyaddress.it ([SERVERIPADDRESS])
by mc2-f26.hotmail.com with Microsoft SMTPSVC(6.0.3790.211);
Thu, 23 Jun 2005 03:28:25 -0700
Date: Thu, 23 Jun 2005 12:32:20 +0200
Return-Path: tutor@mycompanyaddress.it
To: mypersonaladdress@hotmail.com
From: Tutor On Line
Subject: Risposta del Tutor on Line
Message-ID:
<26cbeec67ac19df1ce8c58047ab0033e@tutoring.anothermycompanyaddress.it>
X-Priority: 3
X-Mailer: PHPMailer [version 1.72]
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="iso-8859-1"
X-OriginalArrivalTime: 23 Jun 2005 10:28:26.0604 (UTC)
FILETIME=[4A7C6AC0:01C577DE]