sorting office 
noun
An office in which mail is sorted according to its destination.We have told how posties found undelivered mail piling up in sorting offices and how city events that depended on post getting through have been scuppered....
- An investigation has been launched into alleged delays in delivering mail from a major sorting office in Eltham.
- Due to cut backs at Royal Mail our local sorting office doesn't have enough staff to cover for staff who are sick or on holiday.