Signed Debian packages for trixie are available from the PhoenixDKIM APT repository.
Import the release signing key:
sudo wget -O /etc/apt/keyrings/phoenixdkim.asc \
https://www.phoenixdkim.org/phoenixdkim-releases.asc
Add the repository:
echo "deb [signed-by=/etc/apt/keyrings/phoenixdkim.asc] \
https://apt.phoenixdkim.org trixie main" \
| sudo tee /etc/apt/sources.list.d/phoenixdkim.list
sudo apt-get update
Install PhoenixDKIM:
sudo apt-get install phoenixdkim