Police have appealed for information after seven post boxes were damaged in rural areas around Chelmsford on Monday.
The metal Royal Mail boxes are believed to have been damaged when fireworks exploded inside them.
The incidents happened at Stagden Cross and The Street, High Easter, both between 2.30pm and 3.15pm, Bakers Mead, Great Waltham at about 3.30pm, three in Ingatestone Road, Highwood, shortly after 7pm and one at Mill Green Road, Mill Green which was discovered after 11pm.
All the incidents are being linked and investigating officers want to hear from any witnesses or anyone with information about the offenders involved.
Residents in rural areas are also being urged to be vigilant and call police if they see anyone tampering with post boxes or acting suspiciously around them.
Police also want to speak to the driver of a black and yellow Astra seen in High Easter at the time of the incidents.
Any witnesses or anyone with information should contact police at Chelmsford on 101.