Someone voluntarily gives you their email address either online or in person so you can send them emails. They may pick certain types of email content they wish to receive, like specifically requesting email alerts when new blog posts are published. Opt-in email addresses are the result of earning the interest and trust of your contacts because they think you have something valuable to say.
Sample review: “See what happens to your emails after you press send…you’ll know exactly when they opened your email and whether or not they clicked on your link. BananaTag will even tell you whether they’re on their desktop or mobile device. And best of all, your email appears NO different to your contacts. All of the tracking is done seamlessly without any change in the user experience.” — BuzzBlogger
Are you willing to alienate people who disagree with your opinion? Sure, not everyone who disagrees with you will unsubscribe. But if you can’t take the risk of alienating a large portion of them, it’s better to avoid the topic. The only exception are friendly disagreements. For example, it can be a good idea to show your support for a specific sport team, even if you know many in your audience like another team. As long as they don’t take the sport very seriously, it can be just a fun thing to talk about. I could, for example, tell that I’m more of a dog person (we have two dogs) than a cat person. I have nothing against cats, but I like walking with dogs. I doubt almost any cat person will hold that against me.