Moving e-mail

[Andrew]

Does anyone know of a way to move old e-mail out of Mail into another 
folder?
My wife has around 500 read e-mails she wants to keep. It might be 
unrelated, but it's taking longer to open Mail and I thought this would 
un-clutter it and help it open quicker.
Andrew
iMac 20 G4
10.3.9