A dozen postboxes have been stolen from 12 different locations across Wiltshire in the past two months.
Police said thieves were targeting boxes in "quieter, rural areas" and were also stealing the mail inside.
PC Michelle Hayley, of Wiltshire Police, said they may have been stolen for "their scrap metal value or perhaps because they may be collectable items".
She said they were fixed to poles and each one had been "cut free" from the posts.
The first theft was reported in West Lavington on 31 August and the most recent was on Mill Lane in Poulshot on 2 November.
'Expensive to replace'
The force has appealed for anyone who has seen postboxes for sale in "unusual or suspicious circumstances" to get in touch.
A Royal Mail spokeswoman said there were more than 115,000 postboxes across the UK and thefts of them were "relatively rare".
However, she added: "These boxes are expensive to replace and their theft impacts heavily on local communities."