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
user@domain.com zimbraPrefMailForwardingAddress
user@domain2.com
zmprov ma
user@domain.com zimbraMailForwardingAddress
user@domain3.com
zmprov ma
user@domain.com +zimbraMailForwardingAddress
user@domain4.com
zmprov ma
user@domain.com +zimbraMailForwardingAddress
user@domain5.com