Debian and Ubuntu apt
Add the signing key and the repository once, then install.
$ sudo curl -fsSLo /etc/apt/keyrings/postino.asc https://postino.tanis.codes/postino.asc $ echo "deb [signed-by=/etc/apt/keyrings/postino.asc] https://postino.tanis.codes/apt stable main" \ | sudo tee /etc/apt/sources.list.d/postino.list $ sudo apt update && sudo apt install postino