When registering for a signup-enabled node, this message is displayed. However, the email does successfully send.
Also, the message only appears for certain nodes and I haven't been able to track down what it is about certain nodes that is causing the problem.

The log entry for each "failed" send does help:
--
Type mail
Date Thursday, August 20, 2009 - 13:37
User xxxxx
Location https://www.xxx.edu/services/workshops/endnote-basics-wed-08262009-200pm...
Referrer https://www.xxx.edu/services/workshops/endnote-basics-wed-08262009-200pm...
Message Error sending e-mail (from xxxxx@xxx.edu to ).
Severity error
--

Note how it does recognize a User but only logs the from. The 'to' part of the message is blank. So it seems this is generating the error that the message can't be sent even though it is. False positive.

We do use a theme override. Not sure if this helps as it's pretty basic.
--

function ulib_signup_user_form($node) {
  global $user;
  $form = array();

  // If this function is providing any extra fields at all, the following
  // line is required for form form to work -- DO NOT EDIT OR REMOVE.
  $form['signup_form_data']['#tree'] = TRUE;

  $form['signup_form_data']['Name'] = array(
    '#type' => 'textfield',
    '#title' => t('U of XX ID'),
    '#size' => 40, '#maxlength' => 64,
    '#required' => TRUE,
  );
  $form['signup_form_data']['Email'] = array(
    '#type' => 'textfield',
    '#title' => t('E-Mail Address'),
    '#size' => 40, '#maxlength' => 64,
    '#required' => TRUE,
  );

  // If the user is logged in, fill in their name by default.
  if ($user->uid) {
    $form['signup_form_data']['Name']['#default_value'] = $user->name;
    $form['signup_form_data']['Email']['#default_value'] = $user->mail;
  }

  return $form;
}

--

Any ideas why some nodes work and others don't? We do use tokens in our confirmation mails. Not sure that's causing it to bork. It does seem that losing the 'to' is generating the error even though signup is handing off the mail just fine to drupal_mail -> sendmail.
Thanks,
~John