Norton Antivirus offers a feature called “e-mail protection” that I routinely turn off. It’s a service that scans e-mail for viruses as messages come into your system – an appealing idea, but it’s implemented badly. It changes the settings for your POP3 server and frequently prevents e-mail from being retrieved at all. It’s not documented well – an explanatory message should come up when it’s installed. If you turn it off, Norton Antivirus will still scan all files when you open them from an e-mail message, so you don’t lose very much protection. Here’s an article by somebody else who noticed the same problem.