Q: Is it ethical to send email to everyone who visits my Web site, or all of my customers?
A: Funny you should ask... seriously, though, it is often tempting for people to send “bulk email” to large groups of people in order to pique their interest in a new feature on their site, or let them know about a great deal they’re offering. You should resist this temptation. You should never send bulk email to people who haven’t given you permission to include them on your mailing list. Regardless of whether such a practice is ethical, doing so will make you extremely unpopular with a significant number of people.
How do you understand the part in bold?