[stained]

I'm getting emails entitled "Mail Delivery System - Mail Failure - Malformed Recipient Address" like the one below after someone hits the Paid Option button at registration (the free registration option is working perfectly):

A message that you sent contained one or more recipient addresses that were
incorrectly constructed:

DO NOT EDIT THIS USER: missing or malformed local part (expected word or "<")

This address has been ignored. There were no other addresses in your
message, and so no attempt at delivery was possible.

------ This is a copy of your message, including all the headers. ------

To: DO NOT EDIT THIS USER
Subject: Action Required to Activate Membership for xxxxxx Forum
From: "xxxxxx Forum" <webmaster@xxxxxx.com>
Auto-Submitted: auto-generated
Return-Path: webmaster@xxxxxx.com
Message-ID: <20110621230434.0274644a24fd@www.xxxxxx.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 8bit
X-Priority: 3
X-Mailer: vBulletin Mail via PHP
Date: Wed, 22 Jun 2011 00:04:34 +0100

Dear Pending Subscription 3,

Thank you for registering at the xxxxxx Forum. Before we can activate your account one last step must be taken to complete your registration.

Please note - you must complete this last step to become a registered member. You will only need to visit this URL once to activate your account.

To complete your registration, please visit this URL:
http://www.xxxxxx.com/forum/register...0&i=xxxxxxxxxx

<a href="http://www.xxxxxx.com/forum/register.php?a=act&u=1320&i=xxxxxxxxxx">America Online Users Please Visit Here to be Activated</a>

**** Does The Above URL Not Work? ****
If the above URL does not work, please use your Web browser to go to:
http://www.xxxxxx.com/forum/register.php?a=ver

Please be sure not to add extra spaces. You will need to type in your username and activation number on the page that appears when you visit the URL.

Your Username is: Pending Subscription 3
Your Activation ID is: xxxxxxxxxx

If you are still having problems signing up please contact a member of our support staff at webmaster@xxxxxx.com

All the best,
xxxxxx Forum

This is the third so far. Any ideas? I put the 'xxxxx' everywhere by the way. The original emails all have the full activation codes/email addresses obviously.