msgexport.sh: 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/share/roundcubemail/bin/msgexport.sh - from package: roundcubemail

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