Hello,
Last week, our Moodle fall down due to difficulties with our GPFS file system.
It seems that this may cause difficulties with the cron locks that are stored on the file system.
We find it was possible to store cron locks in database, while changing some parameter in the config.php file, as explained here :
https://moodle.org/mod/forum/discuss.php?d=328300&parent=1320576#p1320902
Since one week, we no more encounter email sending problems
Zabelle