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The old POP3 protocol is probably the most popular ISP service for receiving e-mail. Unfortunately it is by default very insecure with regards to how it handles passwords. APOP is an option that fixes POP3's password insecurity, improving overall internet service provider safety and ISP security. Note though that it does nothing for e-mail privacy -- only e-mail security. Something like S/MIME or PGP is required to improve privacy, too. Note also that if SSL is being used, APOP isn't needed.