If you're creating a newsletter in order to show thought leadership the suggestions here about Mail Chimp are spot on.
If you're looking for clients - and expecting them to fill out request for information forms, the sad truth is that most web contact us forms and sign-ups generate more spam as they do actual leads, unless you are in a very narrow B2B market.
The spam bots have successfully broken every single form of attempt to stop them. There is a rumor - probably true based on our experience - that there are factories in China where people are paid 1 cent for every captcha they solve.
The only way to get good, qualified leads is to do it the old fashioned way - a human doing research. We have a full time person that does this.
Companies that offer to sell you big lists of qualified suspects - from personal experience - are a huge waste of money.