Firstly, this was confirmed as legitimate by the NZ prison system, who kvetched up a storm that a friendly letter had been sent.

The Corrections minister says the Christchurch mosque attacks accused should not have been allowed to send a letter to a supporter from his prison cell.
The supporter, based in Russia, posted a copy of the letter on an alt-right website, which has been verified as authentic by Corrections.

A Russian anon posted this on 4/pol/ a few days ago. Many disbelieved him, but some quick analysis showed strong similarities between the writing in the letter and the writing on Brenton's weapons.

Alright retards,here’s the story
A couple of months ago I sent a letter to Brenton Tarrant and today I finally got a response