POP and IMAP are just transfer protocols and don't dictate what the client does with the mail once received. IMAP should always be used for various reason, the

POP and IMAP are just transfer protocols and don't dictate what the client does with the mail once received. IMAP should always be used for various reason, the biggesting being: POP is a real disaster if you have multiple clients trying to use the same mailbox. There is no way to sync message state (read,unread,flagged,replied,...) across clients. Clients randomly delete mail from the server side.