There's only one zimbraPrefMailForwardingAddress (exposed in preferences), however there can be multiple zimbraMailForwardingAddress (the user is not shown their existence but they can be set in the admin console - do note that any bounces to them would reveal the forward though).
zmprov ma email@example.com zimbraPrefMailForwardingAddress firstname.lastname@example.org
zmprov ma email@example.com zimbraMailForwardingAddress firstname.lastname@example.org
zmprov ma email@example.com +zimbraMailForwardingAddress firstname.lastname@example.org
zmprov ma email@example.com +zimbraMailForwardingAddress firstname.lastname@example.org