Guide To Email Bounces: How To Fix Hard Bounce And Soft Bounce

What is an email bounce?

Email servers use SMTP response codes to communicate the status of message delivery. These codes help determine why a message failed:

• Codes starting with 4xx (e.g., 421) indicate a temporary issue (soft bounce).
• Codes starting with 5xx (e.g., 550) point to a permanent problem (hard bounce).

The second digit gives insight into what part of the email process failed:

• X.1.X = Addressing issue
• X.2.X = Mailbox issue
• X.3.X = Mail system issue

The third digit gives you more insight into what caused the bounce. For example, 5.1.1 means the destination email address is invalid (e.g., the username in name@domain.com doesn’t exist). Today, most ESPs report a mix of traditional and enhanced bounce codes, helping marketers troubleshoot delivery problems.

📌 These bounce types can directly impact your sender score and affect the way you send out your campaigns. According to MailerLite’s 2025 benchmark report, the median open rate across industries is 42.35%. High bounce rates can lower this number. Too many hard bounces, in particular, may cause internet service providers (ISPs) to flag your campaigns as spam, reduce your deliverability, or blacklist your domain.

Soft bounce vs. hard bounce in email marketing

What is a soft bounce vs. hard bounce email? Here’s a side-by-side comparison:

What is a soft bounce?

Common causes of soft bounces

So why does an email bounce? These are the most common reasons:

• Full mailbox: If your recipient hasn’t cleared their inbox in a while (or they're on a free plan with limited storage), their mailbox may be full. In this case, your message gets bounced back because there’s simply no room to receive it. This is common for inactive or rarely used accounts.
• Temporary server error: Email servers occasionally go offline due to maintenance, traffic overload, or configuration changes. When this happens, even valid emails can’t be processed, leading to a soft bounce. These issues usually resolve themselves within hours.
• Greylisting: Some email providers use a method called greylisting to filter spam. They temporarily reject messages from unfamiliar senders, expecting legitimate servers to retry. If your infrastructure isn’t set up correctly to handle retries, your emails might not go through.
• Message is too large: Many servers limit the size of incoming messages. If your email includes high-resolution images, heavy attachments, or large embedded files, it could exceed the recipient’s threshold. Anything over 10MB is typically at risk of bouncing.

🤔 What happens after a soft bounce?

Most email service providers (ESPs) will retry sending a soft-bounced email several times over the next few hours or days. For instance, SendGrid retries delivery over a 72-hour window before marking it as undeliverable. Mailchimp performs up to 20 retry attempts. Handling of a hard bounce vs. soft bounce in Marketing Cloud, ActiveCampaign, etc. can vary, so check each provider’s documentation.

✅ How to reduce soft bounces

• Avoid oversized files by compressing attachments and optimizing image sizes.
• Authenticate your domain with SPF, DKIM, and DMARC to avoid greylisting.
• Keep content clean by avoiding spammy language (urgent messaging, “too-good-to-be-true” offers) that might trigger server filters.
• Use a consistent sender name and address to build trust with the servers.
• Monitor delivery analytics to catch soft bounce trends early.

What is a hard bounce?

Common causes of hard bounces

• Invalid email address: These are email addresses that don’t exist, and they’re often due to typos (like “gnail.com” instead of “gmail.com”), outdated contacts, or deliberately fake entries. Continuing to send to these addresses hurts your sender reputation and increases bounce rates.
• Nonexistent domain: Sometimes the domain (the part after the @ symbol) doesn’t exist or is no longer active. This can happen if a business shuts down or a user abandons their custom domain.
• Blocked sender: If the recipient’s email server has flagged your domain or IP as suspicious, your emails will be rejected. This can result from past spam complaints, lack of proper authentication, or frequent mass emailing without engagement.
• Role-based addresses: Emails like support@, admin@, or info@ are often managed by teams and not intended for promotional emails. Many ESPs treat these addresses as risky or non-personal, which increases the likelihood of hard bounces.

⚠️ Why hard bounces are dangerous

Your deliverability can suffer with hard bounces. Your emails may start landing in spam folders or not be delivered at all. Over time, this leads to a damaged sender reputation, as ISPs keep track of your domain and IP history. When too many hard bounces occur, it signals that you're not following best practices.

You could even end up on email blacklists that prevent your messages from reaching inboxes. Finally, email platforms may prevent your campaigns’ success or restrict your account altogether if your bounce rate exceeds acceptable thresholds. Preventing hard bounces is essential to sustaining effective email outreach.

✅ How to reduce hard bounces

• Use a list verification tool. Bounce checker tools like VerifiedEmail scan your email list in advance and flag invalid addresses, reducing the risk of hard bounces. Consider also testing email deliverability.
• Implement double opt-in. This makes sure new subscribers confirm their email addresses before receiving messages, helping filter out fake or mistyped entries early.
• Remove hard-bounced email addresses immediately after each campaign. Use your ESP’s bounce reports to identify them. Continuing to send to known hard-bounced addresses will damage your reputation.
• Don’t buy email lists. Purchased lists often contain stale, inactive, or even malicious addresses. Even if legal, they typically result in poor engagement and high bounce rates.
• Avoid role-based addresses unless opt-in is confirmed. Only keep addresses like support@ if you’re certain someone personally opted in. Otherwise, they’re high-risk and should be filtered out.

Check if your email is valid with VerifiedEmail.

Verify

Best practices to prevent email bounces

Deliverability problems often stem more from your list quality than your subject line. If your audience didn’t ask for your emails (or if their addresses are out of date), your messages are more likely to bounce.

📌 To prevent this, avoid buying lists and use a double opt-in process. Not only does this confirm that recipients actually want your emails, but it also helps weed out typos, fake signups, and bots. Once your list is clean, take the time to segment your subscribers. This ensures you’re sending relevant content based on the behavior, interest, or engagement level of your mailing list.

📌 Consistency is another factor. Sudden spikes in send volume or long gaps between campaigns can trigger spam filters. A steady, predictable sending rhythm helps you stay in good standing with inbox providers. And before you hit send, always run test sends with a small group to catch issues early.

📌 You should also address the technical side. Implement SPF, DKIM, and DMARC records to prove your legitimacy to recipient servers. This trio helps authenticate your domain and strengthens your sender reputation over time.

📌 To ensure your messages actually get delivered, use tools to test and improve email deliverability before each major campaign. Clean your list regularly with a trusted email list cleaning service like VerifiedEmail to identify and remove invalid, inactive, or risky addresses. Lastly, evaluate your sender reputation using tools like a bounce checker or sender reputation checker.

Lower your bounce rate, protect your email campaigns

Most bounce-related issues are preventable. So marketers must stay on top of list hygiene, verify new signups, warm up new email addresses, and watch for deliverability red flags. These practices make a significant difference not just in metrics, but in where your emails actually land.

Regular bounce monitoring helps keep everything running smoothly. Whether you're launching a re-engagement campaign or building a fresh list from scratch, the goal is the same. You need to send emails to people who can (and want to) receive them.