exmh: 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/exmh - from package: exmh

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

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