In reply to nchakravarthy:
First, I strongly recommend that you always parameterize mailboxes with a specific type. That way you get a compiler error if you try to put or get something from your mailbox of an incompatible type. Unparameterized mailboxes are left-over from a language that did not have parameterized classes and there is no other feature in SystemVerilog that lets you pass an untyped argument to a class method.
The feature that returns a negative number is later addition to the SystemVerilog standard (1800-2009?). You should get a later version of your tool, or contact your vendor.