A string of postbox thefts could have been for their heritage value and to ship them overseas, police said.
Four were stolen in Cheshire last year from rural areas in the south of the county.
PC Jonathan Lightfoot, a rural crime officer with Cheshire Police, said the thefts were not only a crime but meant "part of history had left".
More than 110 post boxes were stolen across the country last year, police said, and a Royal Mail spokesperson said they were working with the charity Crimestoppers to encourage people to share information on such thefts.
"We think it's either for order or for people to display outside their houses because they're quite ornamental," Lightfoot said.
"We might think they might be going abroad as well.
"Now people have stolen those postboxes, they're gone, that history has left."
The latest theft, near the village of Tushingham, close to Whitchurch, just over the Shropshire county border, saw the front door ripped off a box and the pole left at an angle.
The back of the box remained and a member of the public retrieved and posted the mail in another box.
"The door is the more ornamental thing, it's got the crest on it, the initials of the monarch, so that's probably the bit they do want - the box behind is an added extra," Lightfoot said.
"It's a bit of an unsuccessful mission because they haven't got it out of the ground and I think that's because of how solid it is in the ground.
Chris Taft, collections manager at the Postal Museum, said postboxes were part of the country's "cultural identity" and much loved.
"It's a little bit of history in the streets and because they do carry the Royal cipher, you're able to date them," he said.
"There's very few pieces of street furniture left on the streets that might have been there in Victorian England and are still in use."
He added the thefts of postboxes were "horrible".
"It's destroying parts of our heritage and I think it's awful that people feel the need to steal these things, often damaging them in the process," he said.
"It's not just taking away the local facility, it's destroying an important part of history that has sat there for over 150 years in some cases."
PC Lightfoot urged communities to "keep an eye on" postboxes they thought might be vulnerable and report any concerns to the police.
He said they were working with Royal Mail, using new technology to try and prevent further crimes.
"We're looking at things like forensic marking, we're looking at other methods as well.
"We're trialling 3D scanning software where we'll build a 3D model of the postbox and then we'll have a record of what it looks like.
"So if we find one that is stolen, then we can compare it to records of ones we've already scanned and prove that that's from there."
The spokesperson for Royal Mail said, while postbox thefts were "relatively rare", if their staff noticed a series of them being stolen, they worked with local police to tackle the issue.
"We appreciate how important access to a postbox can be for a community, and we always aim to balance service provision with practical constraints," they added.
"Across the UK, we work with Crimestoppers to encourage people to come forward to any details of postbox thefts."
Postboxes in the UK
The first postboxes in the UK were installed on the Channel Islands, after Post Office clerk - and later novellist - Anthony Trollope proposed their use after seeing them in continental Europe.
Boxes began being rolled out on the UK mainland from 1853.
Early postboxes were green but they were standardised red in the 1870s.
Each box has the cipher of the monarch who was on the throne at the time.
The first King Charles III box was unveiled in Cambridgeshire in 2024.