Collins was arrested in May 2013 for carrying two knives in a Japanese department store. He was detained at a Hamamatsu police station following his arrest, and Campbell went to see him.
"Hey john id just like to say thankyou for coming to see me while i was in jail in japan," Collins wrote.
"You were the only journalist who came and though we couldnt physically speak through the glass, the sign language helped eliviate my families fears."
Collins said Campbell was one of few "great people" in the world and said God loved him.
He signed off wishing Campbell well. "Much love from your friend playing rugby again in the south of france."
The email comes as footage emerged of Collins doing the haka the day before his death:
Collins's agent, Tim Castle, said this morning that the player's daughter, Ayla, remained in critical condition in a Montpellier hospital and the outlook was uncertain. "That's the most accurate word I can give you at this time," he told Radio NZ.
Collins' family and friends have gone to Montpellier to be beside Ayla.
More than $13,600 has been raised on the Jerry Collins Memorial Fund Givealittle page established for Ayla.
Prime Minister John Key paid tribute to Collins and Ms Madill this morning.
"It's very sad," he said on TV One. "He's someone who earned our respect on the rugby field."
