Two letterboxes in Blenheim were set on fire at the weekend and police are now appealing to the public for information.
The first letterbox was set on fire overnight on Friday in South St, and the second letterbox was set on fire overnight on Saturday in Covent Garden, police said.
Accelerants were used in both incidents.
CCTV footage of the two locations was being reviewed, police said and items of interest had been recovered from both scenes which would now be examined.
It was unknown whether the two incidents were linked, police said.