In most cases, people ignore the verification of their email database because they don't realize the impact of sending emails to wrong addresses.
The usual attitude I come across is, "Even if 20%-30% of the email addresses are invalid out of the database I have, I will still make a profit." They don't realize that the "20%-30%" is enough to damage their sender reputation. If you are one of these people, start with verification of your email database.
Otherwise, all of the steps mentioned above are equally important — make sure you follow all of them before sending out your next email marketing campaign.