If IMAP on a computer is set to download messages, am I correct in assuming it saves a copy on the computer and doesn't have to start downloading from scratch every time the computer is switched on.
ie. if there are 10,000 emails on the server and the computer is set to download them (for this example pretend it is necessary), once it has downloaded the 10,000 , do they stay on the computer as well as on the server , and next time the PC is restarted it doesn't have to freshly download them all again ?
IMAP e-mails normally remain on the server until they are purged (deleted). My IMAP account only downloads new e-mails. I use 2 PCs and a smartphone with my IMAP and I can have my e-mails at each location. On one of my PC's I archive old e-mails and then purge the server so that the amount held on the server doesn't become too large. Your provider should have some guidance.