Though there are problems with security questions, they are the best way we have of authorizing access to your account if you lose access to your forwarding address, or you have a Mailstore account. To avoid letting acquaintances gain access to your account, we do not use standard security questions like "What is your mother's maiden name?" Pobox allows you to set your own security question.
A good security question is one where it would be difficult to know, look up or guess the correct answer. A good security question should also have an answer that stays the same over time. So, "What street do I live on?" is a bad security question. "What was my bike's nickname when I was 7?" is a good security question.