sendmail.sendmail: 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.sendmail - from package: sendmail

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

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