To receive email alerts if a validator becomes unresponsive or out-of-date, sign up with the Avalanche Notify tool: http://notify.avax.network.
Avalanche Notify is an active monitoring system that checks a validator's responsiveness each minute.
An email alert is sent if a validator is down for 5 consecutive checks and when a validator recovers (is responsive for 5 checks in a row).
When signing up for email alerts, consider using a new, alias, or auto-forwarding email address to protect your privacy. Otherwise, it will be possible to link your NodeID to your email.
This tool is currently in BETA and validator alerts may erroneously be triggered, not triggered, or delayed. The best way to maximize the likelihood of earning staking rewards is to run redundant monitoring/alerting.