Legal analysis: A handwritten letter of guarantee will be established and legally effective once it is signed and sealed, provided that it does not violate the mandatory provisions of the law and is a true expression of intention.

Legal basis: Article 146 of the "People's Republic of China and Civil Code" Civil legal acts performed by the actor and the counterparty with false intentions are invalid. The validity of civil legal acts concealed by false intentions shall be handled in accordance with relevant legal provisions.