推荐的安全措施，以防止意外地向客户发送垃圾邮件

I'm working on a site in which users can opt-in for email notifications for various events (think Google Alerts). An early version of the site had a problem that accidentally generated hundreds of emails to the same few test users. That bug has since been fixed, but leads to the question: what are some recommendations for ensuring that customers won't accidentally be bombarded with email? (Aside from ensuring that the code is bug-free!)I'm working on a site in which users can opt-in