Within the default interface of the SMF software there is no way an administrator or anyone else can read your personal messages. Only you and the person you sent it to are able to read what you wrote. However, if a person has access to the database itself, then it is possible for such a person to read personal messages by searching for them in the database with an SQL command. For this reason, it is advised to not send personal messages that contain private information that you do not wish for anyone else to read.