I know howto create a sender filter so no one would receive the ex-employee's email. But is there a way to have all the ex-employee's emails that he/she's trying to send to us to instead all go to our one manager'sinbox for legal reasons? Thanks

You can create a Sender filter using the E2K3 or E2K7 anti-spam features so that the ex-employee's e-mail address is filtered. However, nothing stops them from changing their e-mail address. If you are using E2K7, you could use a Transport Rule to redirect all inbound mail from a certain address to a mailbox. In my opinion, it is probably better to send the ex-employee a "cease and desist" letter along with a hint of legal meance tone. If he/she does not stop, call it harrasment and call the police.