You can only use one mail client at a time. The mail files are locked by the mail program that is currently using them, so the mail files can't be accessed by any other mail program.
Posta-bezero bat bakarrik erabil dezakezu aldiro. Posta-fitxategiak unean erabiltzen dituen posta-programak blokeatuta daude, beste posta-programa bat posta-fitxategietan sar ez dadin.