Can't cancel CanUseMailbox
API says for CanUseMailbox:

CanUseMailbox
  • Called when the player tries to use a mailbox
  • Returning true or false overrides default behavior
true or false dont do anything different. They still open the mailbox. Am I missing something?

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?