The Royal Mail and the Communication Workers Union (CWU) are appealing for owners to keep their dogs under control as over 3,100 workers were attacked between April 2011 and April 2012.
As a result, 90,000 posting pets have been issued by the Royal Mail, which are devices used to put letters through the letterbox that protect workers' fingers.
Over the school holidays, postmen and women are at a higher risk because children are at home and dogs are sometimes left unsupervised in the garden.
Dave Joyce, CWU national health and safety officer, said: "The age old image of the dog attacking the postman is not a laughing matter.
"Thousands of our members are bitten every year and hundreds suffer debilitating injuries every year which leave them with physical and psychological scarring, some with life changing disabilities and all in the course of doing their job."
Ian Stockdale, head of health and safety management for Royal Mail, added that while most dogs are not dangerous, even placid animals can attack if they think their territory is being threatened.
Tips for keeping dogs under control include keeping dogs indoors at the time post is being delivered, and keeping them in another room or a cage if they are prone to attacking mail in order to protect workers' fingers.