The problem is the “www-data@store” address that apache is trying to send the email from. I run into this all the time and there is always a different solution. Usually it is a programming issue.

Basically, you have to make sure that the ‘From’ address for the email is a valid and locally resolvable email address. So, depending on what software you are using to send the email the fix will vary.

If it’s something you have developed in PHP yourself, make sure you set the 'From’ header in the $additional_headers parameter. eg:

$from = 'you@example.com';
$additional_headers = array('From' => $from);
mail('someone@else.com', 'Test Email', 'This is the content', $additional_headers);

If that doesn’t work, you can also try setting it as an additional_parameter and send the sendmail -f argument that will explicitly set the from address. This is usually a catchall solution. eg:

$from = 'you@example.com';
$additional_headers = array('From' => $from);
mail('someone@else.com', 'Test Email', 'This is the content', $additional_headers, '-f ' . $from);