tmda-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/bin/tmda-sendmail - from package: tmda

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

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