Never, ever, resort to 'purchased' lists, or otherwise send email to people who have not explicitly opted in to receive your email - this will backfire on you in no time. If even a handful of people receiving your email mark it as Spam, email service providers (like Gmail, Yahoo etc.) will readily blacklist you. A blacklist means you will not be able to send emails to any email address with that service provider. There are also specialist spam watch groups - like SORBS, Spamhaus etc. - that are dedicated to identifying IP addresses and mail domains that send spam. Service providers have also grown very sophisticated over the years in tracking spam, so do not try to cheat them by changing your identity - it is only a question of when you are going to be caught - not if. It is also against the law to send unsolicited email.
Get a report of your sent mailings and ensure that any email address marked as a 'hard bounce' - meaning, a permanent error in the address - is removed from your future mailings. Sending to hard-bounced email addresses is considered by service providers as an indicator of spam - you don't want to go there.
At the point of collecting users' email address, a web form for example, have the users type in their email address twice - and validate that they match - to ensure you don't get invalid email addresses by mistake, that add to your bounce rate.
Another possible outcome of resorting to purchased email addresses is the inclusion of 'spam trap' addresses. These are addresses used by service providers and blacklist watch groups. If you send emails to such email addresses, you will very quickly get into their blacklist.
Thus, when it comes to effective mailing lists, the clichéd mantra of quality matters more than quantity, still holds true.