How to send mail when F5's CPU usage is 80%.

Presumably you want to receive the log as at https://support.f5.com/csp/article/K67045449

You need to find the alert name from the Log ID number which are stored in the map files in /etc/alertd. eg in this case you can see that the alert name is BIGIP_SYSTEM_CHECK_E_CPU_TEMP_HIGH and the log ID is 010d0001. So, look for the actual log entry used, pick out the ID and use 'grep /etc/alertd/*' to find the alert name and use that in your user_alert.conf file.

[root@asm1:Active:Standalone] alertd  grep BIGIP_SYSTEM_CHECK_E_CPU_TEMP_HIGH *
alert.conf:alert BIGIP_SYSTEM_CHECK_E_CPU_TEMP_HIGH {
bigip_system_check_error_maps.h:0 LOG_EMERG     010d0001 BIGIP_SYSTEM_CHECK_E_CPU_TEMP_HIGH     "Blade %d Cpu %d: temperature (%d) is too high."