claws-mail: 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 CentOS possible default full paths are:

/usr/bin/claws-mail - from package: claws-mail

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/claws-mail - from package: claws-mail

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

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