Is email marketing illegal? I have this method..

So I have a referral, and my goal is to get people to sign up for it. It's in a specific niche that I can target easily by getting people's yahoo emails through Yahoo! Answers, but I'm wondering if it's illegal to send a referral link to someone who's email they didn't give me. I won't spam them, it'll probably be just a one-time email.

I believe you need to include a physical address and opt-out link as well as indicate it is an advertisement to comply with the spam act. If it's a one-time email, I don't think you'll need to worry about any legal action though if you don't include the above.

2 things here:
1- The offer you are promoting may not allow you to spam
2- If they didnt give you their email it is spam

That doesnt mean it cant be or isnt done