getmail_maildir: 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/getmail_maildir - from package: getmail

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

On Debian possible default full paths are:

/usr/bin/getmail_maildir - from package: getmail4

You may install the required package with command apt-get install {package}

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