esmtpd-ssl: 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/esmtpd-ssl - from package: courier-mta-ssl

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

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