Vandals destroyed or broke into nine post boxes in Wellingborough during a weekend rampage.
Royal Mail said several of the post boxes were damaged beyond repair and will have to be replaced, while others are currently awaiting repairs.
Northamptonshire Police said it had also been informed about the break-ins.
A spokesman for Royal Mail said: “Over the weekend, nine postboxes were broken into in the Wellingborough area. Three boxes were destroyed beyond repair, and will be replaced as soon as possible.
“The remaining six boxes are being repaired this week.
“In the meantime, customers can use other postboxes in the area or of course drop their mail off at any Post Office branch.
“We have some 115,000 post boxes around the country and thankfully cases like these are relatively rare. Royal Mail urges anyone with any information to contact the police.
“Royal Mail apologises to any of its customers who have been inconvenienced or who may have had items of mail stolen.
“Boxes are cleared six days a week and we would advise anyone who has any concerns about their mail to contact our customer services team on 08457 740 740.”
Royal Mail said the full list of post boxes affected is: Westminster, Edwards Drive and Owen Close (these three post boxes will be replaced); Burns Road, Olympic Way, Northern Way, Shelley Road, Minerva Way and Fallowfield (these will be repaired this week).
Anyone with information is asked to phone Northamptonshire Police on 101.