Unable to receive alerts on email

We are using EBS 12.2.10 with database 19c (19.10.0.0.0) we have configured some alerts in system which are used to send details on specific emails within company, not outside. Suddenly all alerts have been stopped sending emails. We checked email server there is no restriction or firewall. We checked queries of every alert and it is showing results on SQL developer. There is no change in system and all alerts are enabled. Concurrent request of every alert also completed successfully, there is no error or warning. We need to know how it can be troubleshoot to find root cause of