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

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