Email

Use the standard email_to_user function and hack its arguments to do more advanced stuff. ATTENTION: adding cc and bcc headers in this way only works if you're not using smtp.

Here's how you add a Bcc header:

        $from = generate_email_supportuser();
        $from->customheaders = "Bcc: mysecret.address@solin.eu";
        email_to_user($user, $from, $subject, $messagetext);

Update: use core_user::get_support_user();. As a matter of fact, it looks as though the customheaders should be an array now:

        $from = core_user::get_support_user();
        $from->customheaders = array("Bcc: mysecret.address@solin.eu", "Cc: onno@solin.nl"); 
        email_to_user($user, $from, $subject, $messagetext);

Use html_to_text to create a proper plain text version out of your html email. Example:

email_to_user($user, $supportuser, $subject, html_to_text($messagehtml), $messagehtml);

Please note that the $user's property mailformat determines the email layout. If $user→mailformat == 1 then the html format is used, otherwise plain text. The site wide user preference default is can be retrieved through $CFG→defaultpreference_mailformat. If you use $user = core_user::get_user($user_id), then the $user will contain the mailformat property.

Sometimes you have to send e-mail to users who don't exists yet. Instead of creating a dummy user object, there is a some simple functions who can do that for you, which are called user_picture::fields() and username_load_fields_from_object().

Usually creating a dummy user object goes something like this:

    $postuser = new stdClass;
    $postuser->id        = $post->userid;
    $postuser->firstname = $post->firstname;
    $postuser->lastname  = $post->lastname;
    $postuser->imagealt  = $post->imagealt;
    $postuser->picture   = $post->picture;
    $postuser->email     = $post->email;

But you can make it easier by using the other functions to create a simple user object from the information you probably already have.

   $to_user = new stdClass();
   $userfields = explode(',', user_picture::fields());
   $to_user = username_load_fields_from_object($to_user, $invitation, null, $userfields);
   $to_user->id = -1;