Royal Mail has confirmed that between 10,000 and 11,000 postal items were recovered by police following an arrest earlier this year.
The huge haul of mail is now being painstakingly sorted by hand before being sent on to addresses across the country.
The find was made at a property in Bury, with Royal Mail confirming that the person involved is no longer employed by the business.
The scale of the operation means it could take considerable time before every item is returned to the postal system.
Each piece of mail must first be sorted into delivery areas and matched with a covering letter explaining the circumstances of the delay.
Some of the recovered post was also found to have been damaged and is being dealt with separately.
The discovery has sparked questions about how such a large quantity of undelivered mail came to be stored at a single property, with thousands of letters and other postal items now being processed after being recovered by police.
Royal Mail says its teams are focused on ensuring the recovered items are returned to their rightful recipients wherever possible.
A Royal Mail spokesperson said: “We take the security of mail very seriously.
“Following an arrest earlier this year, police found a quantity of mail at a property.
“Our teams have been working to recover and process the items so they can be returned to the postal network and delivered to customers.”
The postal service confirmed it was made aware that police had recovered between 10,000 and 11,000 items following an arrest in the summer.
The mail is currently being sorted by delivery area, with every item requiring manual handling before it can be delivered.
Greater Manchester Police has been approached for comment.