sendmail.postfix: 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/sbin/sendmail.postfix - from package: postfix

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

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