Wordpress new registration email (ONLY) blocked by gmail

Question

The Problem: All Wordpress registration emails (with the password) are getting blocked by google.

Details:
The site's email is currently being hosted by google apps. THOSE emails WILL send to gmail addresses. So the problem is specifically emails generated by Wordpress.

What I've tried:
I've installed several SMTP plugins, and they all allow me to generate test emails (via the plugin settings page) to my gmail account. However, they do not resolve the issue for new user registration emails.

I've also attempted to change the "from" email settings, but I'm not really sure how to do this. You can see in the email header pasted below, that I'd probably need an address that ends with @performancehosting.axiom9.com for this to make any difference.

Review:
It sounds like wp_mail() is the problem, I'm hoping someone can offer a solution. My suspicion is that it has something to do with the return-path in the email's header, but I'm not sure what to do with this information.

Here is the header from the email generate by Wordpress (which won't arrive at gmail addresses):

Subject:
[Street Of Walls] New User Registration
From:
Street Of Walls <info@streetofwalls.com>
Date:
9/8/11 6:24 PM
To:
e@thesecretaddress.com
Return-path:
<info@streetofwalls.com>
Envelope-to:
e@thesecretaddress.com
Delivery-date:
Thu, 08 Sep 2011 16:24:51 -0600
Received:
from mail-qw0-f46.google.com ([209.85.216.46]) by box780.bluehost.com with esmtps (TLSv1:RC4-SHA:128) (Exim 4.76) (envelope-from <info@streetofwalls.com>) id 1R1n1f-0002NF-Ol for e@thesecretaddress.com; Thu, 08 Sep 2011 16:24:51 -0600
Received:
by qwb7 with SMTP id 7so843432qwb.19 for <e@thesecretaddress.com>; Thu, 08 Sep 2011 15:24:50 -0700 (PDT)
Received:
by 10.224.78.146 with SMTP id l18mr1081350qak.81.1315520690356; Thu, 08 Sep 2011 15:24:50 -0700 (PDT)
Received:
from www.streetofwalls.com (performancehosting.axiom9.com. [69.175.2.122]) by mx.google.com with ESMTPS id el2sm4364671qab.2.2011.09.08.15.24.49 (version=TLSv1/SSLv3 cipher=OTHER); Thu, 08 Sep 2011 15:24:49 -0700 (PDT)
Message-ID:
<f4927d34a6faf41bd2c8514296228fc3@www.streetofwalls.com>
X-Priority:
3
X-Mailer:
PHPMailer 5.1 (phpmailer.sourceforge.net)
MIME-Version:
1.0
Content-Transfer-Encoding:
8bit
Content-Type:
text/plain; charset="utf-8"

New User Registration on your site:
Street Of Walls

User ID: 69
Username: emerson
Email: e@thesecretaddress.com
IP Address: 132.216.228.76

Answer 1

It turns out that there was a plugin which was probably to blame. I changed the site's configuration to use the default Wordpress "new user" email and it suddenly worked. It would still be interesting to understand more about the interface between Wordpress and PHP_mail(), but my problem is solved for now.