mail-expire: 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/bin/mail-expire - from package: mail-expire

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

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