Good data is essential to email marketing success and takes the form of valid email addresses. Having good data mean there is a better chance to turn that data into quality leads, saving time and money. Actively exercising good list hygiene and keeping visitor opt-in forms on your website short are important steps to keeping, and gathering quality data. 

Bad data comes in the form of invalid email addresses. People changing their jobs, internet service providers and names often causes email addresses to become invalid, which leads to bounced emails. High email bounce rates can tarnish your sender reputation and have you labeled as a spammer.

To promote good list hygiene it's a good idea to look over obtained email address lists to see if the email address' appear to be legitimate or you use a data verification service to confirm email address' are indeed real.