Many people like to keep the BeMail status window visible so they can see the communications between BeOS and the POP server. Unfortunately, the status window isn’t minimizable, and is in the way most of the time.
Rather than showing it always, just click in it and hit Alt+W to close it. Next time you check your mail (say, by right-clicking the mail icon in the Deskbar) the status window will spring back to life. This way you can just monitor communications when they’re happening, and keep that window hidden the rest of the time.