The driver, a 24-year-old man from Chelmsford, was arrested on suspicion of throwing the unauthorised package over Chelmsford Prison wall earlier this month.
He was released on bail until November 11, pending further enquiries.
Police are working with the Ministry of Justice and Chelmsford Prison to tackle the issue of people throwing unauthorised packages over the prison walls.
Anyone caught is committing an offence under Section 79 of the Serious Crime Act, which could lead to a prison sentence of up to two years.
Anyone who sees people acting suspiciously is asked to contact police on 101.