If mail has been successfully sending from the application (new question alerts, mail about a new comment, weekly activity reports, etc.) and suddenly stops working, your deploy account password on the linux server may have expired and the cron jobs have stopped running.

Once you change the password, you can start/stop/restart the cron jobs with the following commands:

Start cron service

/etc/init.d/crond start -or- # service crond start

Stop cron service

/etc/init.d/crond stop -or- # service crond stop

Restart cron service

/etc/init.d/crond restart -or- # service crond restart

To check if you've successfully restarted the cron service, you can check an application queue like the mail queue. You'll likely see many backed up items there that will clear out within the next hour as the cron jobs run.