Knowledge base

The difference between hard and soft bounces

When an email bounces, it means that it couldn’t be successfully delivered to the recipient. As a result, the email will “bounce back” to the sender. Bounced emails can be either hard or soft.

In this article, we’ll explain the difference between hard and soft bounces, why they occur and what you can do to avoid them, as well as what to do with those in the On Hold list.

Hard bounce

A hard bounce signifies that the recipient address can’t receive emails for a permanent reason. Hard bounces are automatically added to your suppression list and no further delivery attempts will be made.

Here are some of the most common reasons an email will hard bounce:

  • The email address doesn't exist - The inbox is either no longer active or never has been (the email have been submitted with a typo, for example, instead of

  • The recipient's server is blocking the email - Recipients using business, educational or government email addresses often have much stricter spam filter settings which can result in emails hard bouncing.

Soft bounce

A soft bounce refers to an email that reaches your recipient's mail server but bounces back undelivered before reaching the inbox. In that case, MailerSend will retry to deliver the email. After several unsuccessful attempts, MailerSend will quit attempting delivery in order to maintain your sending reputation.

A soft bounce is a temporary deliverability issue, as opposed to a hard bounce, which is permanent.

Some possible reasons for soft bounces may be:

  • The recipient's mailbox is full - Any further attempts to send emails will result in soft bounces

  • Server timeout - The recipient's server may be under maintenance or temporarily unavailable for other reasons

  • The email is too large - Some inboxes may disallow emails with attachments above a specific size

  • The email may be blocked due to the content - This may happen due to the receiving server's anti-spam or anti-virus filters

On Hold

The On Hold list will be populated by emails that soft bounced 5 times within the last 30 days. Any email that matches this criteria will be added to the blocklist for 72 hours, and after it will be whitelisted. If the email address hits 5 soft bounces in 30 days again, it will be put back on the On Hold list for 72 hours.

Keep in mind, emails cannot be sent to addresses in the blocklist.

How to reduce email bounces

Bounced emails (particularly hard bounces) can be extremely damaging to your reputation as a sender. To maintain a healthy sender reputation, bounces should be avoided at all costs. Some things you can do to avoid email bounces include:

  • Validate email addresses: Use our email validation tool to make sure the email addresses you have are active to stop a bounce before it happens

  • Use suppression lists: These lists help you keep track of bounced emails, complaints, people who don't want emails, and block emails that look suspicious

  • Stop Bad Emails from the Start: Use a reCAPTCHA and a double-check on the email address field when people give you their email. This can catch mistakes or stop spam from getting into your list.

  • Enable double opt-in wherever possible

  • Remove hard bounces immediately from your contact list

On top of making sure your emails aren’t being sent to undeliverable addresses, optimize your emails to avoid spam filters. You can do that by using images and links sparingly and avoiding spam-like phrases such as:

  • Act now

  • Earn cash

  • Offer expires

  • Best price

  • Bonus

  • Cheap

  • Free

  • Discount

Need more info?

Please feel free to contact us at and a member of our support team will gladly help you.