sendmail.opensmtpd: 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.opensmtpd - from package: opensmtpd

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

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