ExceptionReply is a Published TIdReply property that represents the response used when an exception occurs after signalling the associated command object in Check. When an exception is raised, ReplyExceptionCode is used with the message text from the exception as the new value for Reply. The peer thread connection in the associated command object is used to send the reply using SendReply when PeformReply indicates that a reply is needed. 

An exception that occurs in Check can result in the OnException event handler for the server context being executed. Check examines the ReplyExceptionCode on the server to ensure that either condition is detected.

TIdRFCReply 

TIdCommand 

TIdCommand.SendReply 

TIdCommand.PerformReply 

TIdTCPServer.OnException