API says for CanUseMailbox:
CanUseMailbox
- Called when the player tries to use a mailbox
- Returning true or false overrides default behavior
Could you show an example of how you are using it?
Some things are handled somewhat client-side regardless of what the server says, which could possibly be the case here.
After playing with it further, I think I misunderstood what the intention was. I beleive it is working as intended. Sorry.
What exactly were you expecting?