Final answer:
A Broker-Dealer can hold up a client's mail based on the client's written instructions in certain situations, such as travel, ensuring regulatory compliance and preventing fraud.
Step-by-step explanation:
Upon written instructions from a customer, a Broker-Dealer (BD) may hold up a client's mail for a reasonable period under certain circumstances. The BD is usually allowed to do this in situations such as when a client is traveling or if they will not be at a known address to receive their mail. It is important for the BD to ensure that the instructions are clear and to comply with regulatory requirements to safeguard against potential issues like fraud. Holding mail should be done in a way that does not hinder the client's access to important documents or information regarding their account.