OnAlert is a TIdAlertEvent property that represents the event handler triggered when the IMAP4 ALERT response code is retrieved for a message in an IMAP4 server response. OnAlert is triggered once for each message with the ALERT response code during parsing of server responses in the SelectMailBox and ExamineMailBox methods. 

OnAlert allows the application to present the human-readable text from the server response that contains the special alert associated with the message. Applications must assign a procedure to the event handler to allow responding to the event notification.