mkesmtpdcert: command not found

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

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

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/sbin/mkesmtpdcert - from package: courier-mta

You may install the required package with command apt-get install {package}

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