sylpheed-claws: 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/sylpheed-claws - from package: claws-mail

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

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