Hi, Madhumalar!
In opinion, it is better to use the ME_PROCESS_PO_CUST BAdI, than workflow for this purpose. Here are some details about this.
If you want to use workflow with the "send mail" step, you can create your own workflow.
Just copy the PO Reject Workflow WS65400030, replace the "PO release refused" step with the "Send Mail" step with appropriate binding and activate event linkage.
Bit I'd better use BAdI in this case.