How to implement validations in message mapping?

Hi experts,

I have a simple requirement:

I have a simple synchronous scenario:

SOAP -> XI -> RFC

In a message mapping, one of the fields in the source message is of type String.

However, the target application (RFC) does not expect a non-numeric characters in this string.

My question is how can I perform the validation in XI and tell the sender that this field contains Non-numeric characters?

Please help.

Thanks.

Ron