Typically, if your email list is getting smaller, it is the result of one of two things: Bounces or Opt-Outs.
When you send a blast email out, you will always get a certain number of bounces. The Blue Utopia system crawls through your bounces and anything that was bounced by a code that proves it to be a known bad email address, gets removed from your system. These are referred to as ‘hard bounces’. Soft bounces, on the other hand, are emails that get bounced for reasons such as ‘mailbox full’ or ‘system unavailable’ or ‘unknown.' These bounced emails do not automatically get removed after a single bounce as it is not completely certain they are invalid email addresses. However, the system will remove these after they have been bounced three times.
When a person opts out of your email list, they are removed from your future mailings automatically.