Hello from Finland,
One of our teachers wanted the same function. Just to get an email when a student posts to the forum.
I tried to set it up and run the scheduled task ("Run now") No luck! ( Moodle 3.4.1 (Build: 20180115) )
All I get was the following error. It seems that there is a bug in the code (?)
Forum mailings and maintenance jobs
Execute scheduled task: Forum mailings and maintenance jobs (mod_forum\task\cron_task)
... started 12:26:23. Current memory use 31.7Mt.
... used 3 dbqueries
... used 0.009896993637085 seconds
Scheduled task failed: Forum mailings and maintenance jobs (mod_forum\task\cron_task),Coding error detected, it must be fixed by a programmer: Function cron_setup_user() cannot be used in normal requests!