In most cases, the reason for your hard bounce is likely that the user name does not exist at the domain. If you are following best practices and only sending to users who have directly signed up for your newsletters chances are it is simply because the user made a genuine mistake when giving you their email address. The goal is for you to try to keep your hard bounce rate to less than 2%.
If you are a marketer who acquires your data through various means such as working with list brokers, buying on a CPA basis or via co-registrations you are likely to find higher rates of bogus email addresses that can be damaging your email campaigns. Your sender score is a huge factor in your deliverability success and each time you send to a non-existent address it negatively affects your rating.
Sometimes hard bounces do not only occur because the user’s email address does not exist at the domain. It could be due to the content in your email which cause corporate spam filters to entirely reject your message and kick it back to you. If you want to test if this has happened to you, we recommend you send the recipient a plain text email with general conversational information to see if the server has then accepted the message.
Other problem areas occur when you have too many spam complaints against you, this can cause a major ISP to block you and could result in bounces or messages that never make it to your recipient at all. The blocks put in place by major ISPs such as AOL or Hotmail tend to be temporary blocks which last between 1-3 days but that can be a huge revenue loss for your company in the event that happens.
To keep your data as clean as possible it is best to make sure you implement real time email verification to check to ensure the user name is registered at the domain before you add it to your email list. We also recommend that you work with an ESP that offers bounce handling to automate removing email addresses from your list that have hard bounced in the previous campaign you sent. We also recommend that you regularly clean your entire database quarterly to identify email addresses that have become dormant or to identify account closures.