At least two post boxes in Somerset have been damaged after lit fireworks were pushed inside.
Police said a householder in Launcherly, near Wells, heard a loud bang at about 0230 BST on Sunday. The next morning he noticed a post box nearby was damaged.
A spokesman said a further four boxes had been damaged in Ston Easton, Wells, Haydon, and Upper Milton.
Police think the damage of at least two was due to lit fireworks.
Sgt John Muers said the "senseless" damage deprived rural communities who rely on the post boxes.
The offender was also putting themselves and others at risk of injury, he added.
He said: "We will not tolerate this and are determined to identify whoever is responsible for this pointless behaviour."
Anyone with information is asked to contact the force.