Postboxes across an area ranging from Kilcreggan to the Vale of Leven were smashed open last night.
The Royal Mail says ‘mindless vandals’ were responsible for the damage to 14 boxes.
She added: “Due to the damage caused, all of the postboxes have now been taken out of service and our engineers are currently assessing the damage.
“These acts of vandalism are relatively few but where they do happen Royal Mail takes them extremely seriously.
“Every item of mail is important to us and we would encourage anyone with any information about this incident to contact the police.”
She said there had been a Sunday collection yesterday, when all the boxes were emptied of mail, but anyone with concerns, should call the customer services team on 08457 740740.
Customers should take mail into post offices or post in alternative post boxes.
Ten boxes in the Helensburgh area were attacked, as well as three in Dumbarton and one in Alexandria.
Two boxes in Kilcreggan and two in Cove were wrecked but others in Cove and Rosneath were not attacked.