What must you always check before opening an email from a sender?

The correct answer is to always check the sender's URL email address. This is important because the email address can help you verify the authenticity of the message. Phishing attempts often come from addresses that look similar to legitimate companies but have slight variations, such as misspellings or different domain names. By examining the sender's email address, you can determine whether the email is genuinely from the organization it claims to represent, thus helping to protect yourself from potential scams or malware.

Understanding the other choices provides additional context. Checking the date the email was sent can be useful for understanding the timeliness of the communication, but it does not provide direct insight into the sender's legitimacy. Knowing if you have an account with the sender can be helpful, but that alone doesn't indicate whether the email is safe; legitimate companies may still be targets for spoofing. Lastly, the subject line might give hints about the email's intention but doesn't validate the sender's identity. Overall, verifying the sender's email address is the most crucial step in protecting yourself from harmful emails.