@Klaudia *********
a mule, as in the context that the account is often in a different person’s name to that of the person operating the account and funnelling dirty money (scammed/stolen funds that have not been obtained in a legal manner ) to clean it ( laundering ) and thus making the money appear legally obtained.
A mule/donkey carries things, in this inference, the money is being carried through a bank account dirty and comes out clean.