OnFailedRecipient is an TIdSMTPFailedRecipient property that represents the event handler signalled when one or more of the recipients in an email message fail during transmission using the Send method. 

Arguments passed to the TIdSMTPFailedRecipient event handler specify the details for the event notification, and allow the application to indicate if transmission of the message should continue be or halted. Applications must assign a procedure to the event handler to allow responding to the event notification. 

When OnFailedRecipient has not been assigned for the SMTP component (or when the event handler indicates message transmission should not continue), an invalid receipient email address will result in the message transmission being stopped and an exception will be raised for the failed SMTP response. The event handler code is responsible for handling all aspects of logging or resolving the error condition prior to exit from the method.

Send 

TIdSMTPFailedRecipient