We’ve already touched on the topic of hard bounces and soft bounces in a previous article. Here, we’ll take a closer look at the concept of “bounce” in email marketing—that is, a message that failed to reach its recipient’s inbox. What are bounces? What causes them? How can you avoid them? We explain everything here.
The hard bounce
A hard bounce occurs when an email address is invalid. It’s a permanent issue that can be caused by several factors:
- An address that’s been deleted and no longer exists—this is why it’s important to clean and regularly update your lists (e.g., former employees).
- An incorrectly entered email address (typo or fake email)—this highlights the importance of using opt-in or double opt-in confirmation.
- Blocked by the recipient’s server (due to filtering, poor reputation, etc.).
- Non-configured or expired domain.
- SPF/DKIM/DMARC authentication failure.
Besides these main causes of hard bounce, it could also be due to the recipient blocking the sender’s address, or a misconfigured mailbox on their end.
Keep in mind that hard bounces are not recoverable. Don’t waste time—immediately remove the affected address from your list.
The soft bounce
As the name suggests, a soft bounce is less severe than a hard bounce and offers more flexibility. It refers to a temporary issue that prevents the recipient’s address from receiving your email:
- Their inbox is full and they can’t receive new messages. Note: A full inbox often indicates a rarely used or inactive account. If the recipient never checks it, it’s better to disregard the address.
- Their mail server is temporarily down. In this case, the email can be delivered once the issue is resolved by their provider.
- Your email is too large. Be sure to check the recipient server’s size limits—photos and videos can take up a lot of space.
- Temporary greylisting or DNS issues.
A soft bounce is far less serious than a hard bounce since the issue is temporary and the recipient can often be reached later. However, if the bounce continues, it’s better to remove the address before it turns into a hard bounce.
SMTP: Understanding error codes
Error messages are typically grouped into two main categories:
4xx= soft bounce (temporary).5xx= hard bounce (permanent).
These help guide your actions. A “550 Permanent Failure” = hard bounce, while “421 Server unavailable” = temporary soft bounce.
Why does it really matter?
Bounces negatively affect your sender reputation. A total bounce rate above 2% (hard + soft) starts to trigger spam filters.
Examples:
- Mailreach: soft ~0.34–2.8%, hard ~0.33–2.6%.
- MailerLite: recommends keeping it under 2%.
Other causes of bounce
Depending on the mail server, bounces can be interpreted differently—what’s a soft bounce for one may be a hard bounce for another. This makes the job of email marketers even more complex. Additional factors can also influence bounce rates:
1) Content
With spam being a growing issue on the web, ISPs are increasingly vigilant and block many emails to prevent phishing. Your email—however harmless—might get blocked due to a small error in the content.
To avoid this, carefully craft your message: balance text and images, use full URLs to reputable websites, and avoid trigger words like “free,” “money,” “urgent,” “help me,” etc. Also, pay attention to the subject line: keep it clear, concise, specific, and truthful (no misleading clickbait). Lastly, avoid overusing capital letters, exclamation marks, or question marks.
2) Sender reputation
Your email deliverability also depends on your sender reputation, which builds over time through previous campaigns. To maintain a good reputation and reduce hard bounces, avoid spam traps by regularly cleaning your lists and always follow best practices.
3) Purchased lists
We can’t stress this enough: never buy email lists. They’re full of spam traps and will do more harm than good—leading to a high bounce rate and major delivery issues.
Email marketing best practices: quick checklist
- Regularly verify and clean your list using CaptainVerify.
- Use double opt-in.
- Create lightweight campaigns (small file sizes).
- Monitor soft bounces and remove after X failed attempts.
- Immediately remove hard bounces.
- Keep your total bounce rate under 2%.
To avoid bounces in these cases, it may also be useful to check your email provider’s reputation—if it’s poor, you’ll be affected too. You can also apply for a Sender Score certification. Finally, find the right sending frequency so your recipients recognize your emails without feeling spammed.
