When a new person adds his or her email address to your email marketing list, the person can make a mistake, provide a fake email or the person eventually change to a new email address. If this happens, then your email delivery performance would be affected negatively from this. Your email marketing emails can bounce, land directly to spam folders and even receiver report you as a spammer. Email list verify is the way to avoid this bad situation.
If you are sending your email marketing emails to non-existent email addresses, you will ramp up your bounce rate and destroy your sender score. You should remove all inactive subscribers of your email marketing list filtering out all users that have not opened or clicked your emails in a long time.
According to the research, the fastest way to trigger filtering and blocking on your new IP is the high hard bounce rates. You may want to purchase a service to clean all hard bounces before you start a new email marketing campaign, INBOX provides that service; INBOXVerify. With it, you can verify email lists and see how many bounces your lists have.
Also, most email verification tools can catch duplicates, typos, outdated domains, do-not email records, bogus addresses, and other common user errors.