Shipment change mail problem

When I go to VT02N, enter shipment number GoTo-->Output, entered output type, select medium as 5-External Send , and function as '01' - Shipping point and attached partner.

The Partner has email address, but whenever we add entry for output type and click on Save button it goes it 'Red' status i.e. unsuccesful and did not send a mail to partner.

Please help