We have a Moodle installation with more than 3000 users and 300 courses.
Most users want the forum digest sending feature but it doesn't work well for us.
Every attemp with the cron task hangs without explanation when trying to send digests.
The cron report ends at "Sending forum digests:" line
The normal message sending works well.
After increasing "memory_limit" to 256M and "max_execution_time" to 600 the problem persists. The script uses a lot of CPU and time without success.
Another installation with the same code in the same server using the same SMTP, but with fewer users works well.