The walkout, in Wolverhampton in the UK, started over claims a racist incident had taken place.
Royal Mail said about a third of staff took part in the walkout.
The postal service said it believed staff were back to work as normal the next day. Deliveries in the Black Country suffered some delays.
The walkout is believe to have caused a backlog of 700-800,000 items.
When the night shift started at 2200 BST, some workers joined colleagues staging the protest outside the processing plant.