Hi,
first of all these messages are "only" notices, the module should work fine. You could go to wb backend options and check if Error Reporting ist switched to E_NONE. If not, do so and the messages will disapear.
If you want do get deeper in:
Notice: Use of undefined constant user_id - assumed 'user_id' in /data/www/www.naturefterskolen.dk/w/modules/massmail/modify.php on line 138
Notice: Use of undefined constant user_id - assumed 'user_id' in /data/www/www.naturefterskolen.dk/w/modules/massmail/modify.php on line 121
Notice: Use of undefined constant user_id - assumed 'user_id' in /data/www/www.naturefterskolen.dk/w/modules/massmail/modify.php on line 121
Notice: Undefined variable: users_id in /data/www/www.naturefterskolen.dk/w/modules/massmail/modify.php on line 301
The error message almost says it all

. Edit the file modify.php on lines 121 and 138 and single quote user_id (looks like 'user_id').
Insert new line after line 295: $users_id = "";
This is not perfect as $users_id might be empty in some cases but it's quick.
regards