sendmail: 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
/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 - from package: exim
/usr/sbin/sendmail - from package: opensmtpd
/usr/sbin/sendmail - from package: ssmtp

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

On Debian possible default full paths are:

/usr/sbin/sendmail - from package: citadel-mta
/usr/sbin/sendmail - from package: courier-mta
/usr/sbin/sendmail - from package: dma
/usr/sbin/sendmail - from package: esmtp-run
/usr/sbin/sendmail - from package: exim4-daemon-heavy
/usr/sbin/sendmail - from package: exim4-daemon-light
/usr/sbin/sendmail - from package: lsb-invalid-mta
/usr/sbin/sendmail - from package: masqmail
/usr/sbin/sendmail - from package: msmtp-mta
/usr/sbin/sendmail - from package: nullmailer
/usr/sbin/sendmail - from package: postfix
/usr/sbin/sendmail - from package: qmail-run
/usr/sbin/sendmail - from package: ssmtp
/var/lib/qmail/bin/sendmail - from package: qmail

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