sylpheed: 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

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/sylpheed - from package: sylpheed

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