Why are emails sent from the applications being marked as spam

I have 2 web apps running on the same server. The first is www.nimikri.com and the other is www.hourjar.com. Both apps share the same IP address (75.127.100.175). My server is through a shared hosting company.

I've been testing my apps, and at first all my emails were being delivered to me just fine. Then a few days ago every email from both apps got dumped into my spam box (in gmail and google apps). So far the apps have just been sending emails to me and nobody else, so I know people aren't manually flagging them as spam.

I did a reverse DNS lookup for my IP and the results I got were these:

100.127.75.in-addr.arpa NS DNS2.GNAX.NET.

100.127.75.in-addr.arpa NS DNS1.GNAX.NET.

Should the reverse DNS lookup point to nimikri.com and hourjar.com, or are they set up fine the way they are?

I noticed in the email header these 2 lines:

Received: from nimikri.nimikri.com

From: Hour Jar <invite@hourjar.com>

Would the different domain names be causing gmail to think this is spam?

Here is the header from one of the emails. Please let me know if any of this looks like a red flag for spam. Thanks.

Delivered-To: bapublic@gmail.com
Received: by 10.231.157.85 with SMTP id a21cs54749ibx;
        Sun, 25 Apr 2010 10:03:14 -0700 (PDT)
Received: by 10.151.130.18 with SMTP id h18mr3056714ybn.186.1272214992196;
        Sun, 25 Apr 2010 10:03:12 -0700 (PDT)
Return-Path: <invite@hourjar.com>
Received: from nimikri.nimikri.com ([75.127.100.175])
        by mx.google.com with ESMTP id 28si4358025gxk.44.2010.04.25.10.03.11;
        Sun, 25 Apr 2010 10:03:11 -0700 (PDT)
Received-SPF: neutral (google.com: 75.127.100.175 is neither permitted nor denied by best guess record for domain of invite@hourjar.com) client-ip=75.127.100.175;
Authentication-Results: mx.google.com; spf=neutral (google.com: 75.127.100.175 is neither permitted nor denied by best guess record for domain of invite@hourjar.com) smtp.mail=invite@hourjar.com
Received: from nimikri.nimikri.com (localhost.localdomain [127.0.0.1])
 by nimikri.nimikri.com (8.14.3/8.14.3) with ESMTP id o3PH3A7a029986
 for <bapublic@gmail.com>; Sun, 25 Apr 2010 12:03:11 -0500
Date: Sun, 25 Apr 2010 12:03:10 -0500
From: Hour Jar <invite@hourjar.com>
To: bapublic@gmail.com
Message-ID: <8441961.01272214990893.JavaMail.mailer@nimikri.com>
Subject: brian@hourjar.com has invited you to New Event
MIME-Version: 1.0
Content-Type: text/html; charset=us-ascii
Content-Transfer-Encoding: 7bit

ChrisF, here is the body of the message:

<h1>Hour Jar</h1><h3>New Event</h3><p>To view the event, please go to hourjar.com/event?event.id=2&guest.id=2</p>

Chaitanya, yes every single message from my server is going to spam.

BillThor, thank you for answering my question about rDNS. I will fix the issues you pointed out and see if they help.