Hello!
Currently I have that problem: I am making activity module. After I submit my form and went to the <modulname>_add_instance() function, I wаnt to call email_to_user function to all users that are in the course. But after calling the function nothing is happening.
$subject="test";
$message="testing";
email_to_user($USER, $USER, $subject, $message)
(Тhis is test for calling this function to one user, but still it is not working)
I set up too an SMTP Connection in Moodle, but still i don`t get why this is not working.