squirrelmail-configure: 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/sbin/squirrelmail-configure - from package: squirrelmail

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