dkimverify: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/dkimverify - from package: python-dkim

You may install the required package with command apt-get install {package}
Error in other languages:
dkimverify: bevel nie gevind nie
dkimverify: comandă negăsită
dkimverify: comando não encontrado
dkimverify: commande introuvable
dkimverify: command not found
dkimverify: käsku ei ole
dkimverify: không tìm thấy lệnh
dkimverify: komanda nerasta
dkimverify: Kommando nicht gefunden.
dkimverify: kommandot finns inte
dkimverify: komut yok
dkimverify: nie znaleziono polecenia
dkimverify: níor aimsíodh an t-ordú
dkimverify: no se encontró la orden
dkimverify: no s'ha trobat l'ordre
dkimverify: opdracht niet gevonden
dkimverify: parancs nem található
dkimverify: perintah tidak ditemukan
dkimverify: príkaz nenájdený
dkimverify: příkaz nenalezen
dkimverify: команда не найдена
dkimverify: командата не е открита
dkimverify: 命令找不到
dkimverify: コマンドが見つかりません