HTTP to File with source XML validation

Hey guys

i have a HTTP to File scenario wherein i m getting XML over HTTP from an external system.i need to perform a validation check on this payload and if the check fails then i need to send this payload as a mail attachment to a certain address,if check is successful then i need to post this XML to the File server.

i have done simple scenario without any validation but now i need to upgrade and perform validations,do i need to do that in Message mappin?

some links will be really helpful

thanx

ahmad