Before you hit "send," stop! Just because people gave you their contact information doesn't mean they've opted in to your online newsletter. However, you're halfway there. Turn prospects and customers into e-zine subscribers simply by getting their permission first. You'll grow your e-mail list without putting your business at risk.
First of all, subscribing people to your e-zine without their explicit consent isn't guerrilla marketing--it's spam. And those involuntary subscribers might retaliate. Just a few spam complaints could bring down your website. Or your company could be flamed on a blog that gets top search engine rankings for your company name. Therefore, you must resist the urge to sign people up to your online news-letter just because they have given you their business cards, bought something in your offline or online store, or contacted you through your website.
When done right, however, these are all very effective avenues to grow your e-mail list. If you speak at an event as a way to collect leads, for example, announce during your presentation that people who want to receive your free e-zine should give you their business cards. There. They've knowingly opted in. If you collect cards at an event, ask individuals if they want to subscribe to your e-zine, or send a follow-up e-mail and include a link to your e-zine.
Using double opt-in is a good idea. Once an e-mail address is submitted, an e-mail is automatically sent to the subscriber asking him or her to confirm their subscription. This helps protect your company against spam complaints, and it reminds recipients that they did indeed sign up.