What are some of that firewall can't prevent

1. E‑mail viruses
   E‑mail viruses are attached to e‑mail messages. A firewall can't determine the contents of e‑mail, so it can't protect you from these types of viruses. You should use an antivirus program to scan and delete suspicious attachments from an e‑mail before you open it. Even when you have an antivirus program, you should not open an e‑mail attachment if you're not positive it's safe. For more information, see Avoiding e‑mail viruses.
2. Phishing scams
   Phishing is a technique used to trick computer users into revealing personal or financial information, such as a bank account password. A common online phishing scam starts with an e‑mail message that appears to come from a trusted source but actually directs recipients to provide information to a fraudulent website. Firewalls can't determine the contents of e‑mail, so they can't protect you from this type of attack. For more information, see Phishing Filter: frequently asked questions.