Why Emails Bounce and How to Fix It

Hard Bounces vs Soft Bounces

Understanding the difference is critical because the two types require different responses.

Hard Bounces (Permanent Failures)

A hard bounce means the email can never be delivered to that address. The receiving server has definitively rejected it. Common hard bounce codes include:

• 550 5.1.1 - User unknown. The email address does not exist on that mail server. This is the most common hard bounce and usually means a typo in the address or a deleted account.
• 550 5.1.2 - Domain not found. The domain portion of the email address (everything after the @) does not exist or has no mail server configured.
• 550 5.7.1 - Delivery refused. The receiving server has blocked your sending IP or domain. This can be a hard bounce if the block is permanent.
• 552 5.2.2 - Mailbox full (can be hard or soft depending on the server's implementation).

Every hard bounce address should be removed from your list and added to your suppression list immediately. Continuing to send to hard bounce addresses signals to ISPs that you are not managing your list, which damages your sender reputation for all your emails.

Soft Bounces (Temporary Failures)

A soft bounce means the email could not be delivered right now but might succeed later. The receiving server accepted the connection but returned a temporary error. Common soft bounce scenarios:

• Mailbox full - The recipient's inbox has reached its storage limit. The email may be deliverable once they clear space.
• Server temporarily unavailable - The receiving mail server is down or overloaded. Retrying later usually succeeds.
• Message too large - The email exceeds the recipient server's size limit.
• Rate limiting (421 errors) - The receiving server is telling you to slow down. This is common with Outlook and Yahoo when you send too much volume too quickly.

Most email systems automatically retry soft bounces several times over hours or days. If an address consistently soft bounces across multiple campaigns (3-5 consecutive soft bounces), treat it as a hard bounce and remove it.

What Is a Healthy Bounce Rate

For a well-maintained email list, total bounce rates should stay below 2% per campaign. Hard bounces specifically should be below 0.5%. If your bounce rates are higher than this, you have a list quality problem that needs immediate attention.

• Under 2% total bounces: Normal range. Keep monitoring and removing bounces as they occur.
• 2-5% total bounces: Warning zone. Your list needs cleaning. You may have added addresses without proper validation or you are sending to a stale list segment.
• Over 5% total bounces: Critical. ISPs are likely already penalizing your reputation. Stop sending to your full list and clean it aggressively before resuming.

Common Causes of High Bounce Rates

Old or Purchased Lists

Purchased email lists almost always have high bounce rates because the addresses were harvested without consent and many are outdated. Lists that have not been mailed in months or years will also have accumulated bounces as people change jobs, close accounts, and domains expire. Never buy email lists, and if you have an old list you want to reactivate, clean it with a verification service before sending.

Missing Email Validation at Signup

If your signup form accepts any text as an email address without validation, you will collect typos (gmial.com instead of gmail.com), fake addresses (test@test.com), and bot submissions. Implementing basic format validation, plus double opt-in confirmation, eliminates most of these before they hit your list.

Role-Based Addresses

Addresses like info@, admin@, sales@, and support@ bounce more frequently than personal addresses because they are often unmonitored or have stricter filtering. They can also be spam traps. Some email marketers exclude role-based addresses from bulk campaigns entirely.

How to Fix and Prevent Bounces