Post boxes in Cotmanhay and Shipley View had to be sealed up this week after a set of keys went missing.
Residents were shocked and confused to find that they could not post mail in at least five post boxes in the area - including two right outside Post Offices.
After receiving reports of the blocked boxes the Advertiser set about investigating why the boxes were blocked.
Staff at Ash Street Post Office in Cotmanhay and the post desk at the Summerfield Way Co-op were left baffled when customers told them that they could not post their mail.
Sub postmaster, Jim Manek, said: “We didn’t even know that the box had been sealed up until people started coming in and giving us their letters.
“No one told us why it happened. It’s a mystery.”
Boxes outside the Manor Nursing Home on Kedlestone Drive, Ilkeston and on Church Street and Nelson Street were also closed.
Joy Nottingham told the Advertiser about the problem.
She said: “I’ve been around four post boxes now only to find them all with metal plates over the slot so you can’t post anything.
“There’s no explanation or notice or anything. It’s ridiculous. No one knows what is going on.”
But after the Advertiser contacted Royal Mail admitted the box closures were down to a lost set of keys.
Royal Mail spokesperson Val Bodden said: “Unfortunately we have had to seal a number of postboxes in the Ilkeston area for security reasons due to keys going missing.
“All mail has been removed from the affected boxes and has been processed through the postal system.
“New locks are currently being installed in all the boxes and they should all be back in service by Wednesday.
“ We apologise to customers for any inconvenience while the boxes were out of action.
“No mail was delayed and thankfully this is a rare occurrence.”