Found the Solution. Need to use Split Domain.
zmprov md localdomain.com zimbraMailCatchAllAddress @globaldomainISP.com
zmprov md localdomain.com zimbraMailCatchAllForwardingAddress @globaldomainISP.com
zmprov md localdomain.com zimbraMailTransport smtp:ISPsmtpserver
for further Details
Split Domain - Zimbra :: Wiki