"I know it's not funny that the post man gets attacked by the cat but I couldn't help but laugh at this," Sampson wrote.
The letter also warned that Sampson would be denied mail service if he did not keep the cat restrained or away from the mail slot "at all times" or provide an alternative delivery location within 14 days.
Sampson told the BBC that he believed the postman's story, but hadn't himself witnessed that exact behavior.
"We've noticed over the last couple of days that the postman is very hesitant at putting the letters in and Bella thinks it's a game that he's trying to play," he said. "I haven't seen her put her paws all the way through but I think it's fair what they're saying - it's just how they're worded the letter."
Despite his sympathy for the postman, Sampson stated that he refused to comply with some of the Royal Mail's requests.
"As to restraining the cat - I'd no way dare," he said.