Mailkey user_register_admin_created not sending emails

This is my first core issue so I hope I get it right.

I have a repeatable problem tested on multiple fresh (core-only) installs of 7.21 (and also 7.10 just to confirm). I can use the php mail() function to send emails from my server (Ubuntu 12.04 with Postfix - all very vanilla). Also, I can send emails from within Drupal using the system_action_send_email mailkey (using trigger module and an email action), as well as by the user_cancel_confirm mailkey.

The problem is that attempts to send an email from the new user form at admin/people/create (by checking the "notify user of new account" checkbox") do not result in an email being sent. I have downloaded and installed the "mail_logger" module to try to understand what is happening - as there is nothing in the systems /etc/log/mail.log

I have attached a screenshot of the mail_logger report and would be happy to provide any other information that might help identify this issue.

Thanks

Robert in Oz