How to send custom messages through IDOC

Hi,

In my requirement i am getting some data from third party system into SAP through IDOC.

After I receive that IDOC i need to validate the data . If error data found I need to send an acknowledgment message through IDOC with that error message.

Ex: If i am getting employee number as data into SAP then i need to validate and if it is a invalid employee number i should send a new idoc with message invalid employee number.

Thanking you in advance