The POP3 protocol does not provide any method to send a notification alert to the client mailer. If an Account has any pending Alert message, the CommuniGate Pro POP Module simply rejects the connection request after the user is authenticated.
The returned error code contains the Alert message text:
ALERT: alert message text
When the user repeats a connection attempt to the same Account, the next pending Alert message is returned as an error - till all Alert messages are sent to that user.