Email Bounce Management: Soft Bounces vs. Hard Bounces

April 07, 2024
Written by
Jesse Sumrak
Opinions expressed by Twilio contributors are their own

Email Bounce Management: Soft Bounces vs. Hard Bounces

We’re taking it back to the basics to define a term that is super important to email deliverability but still seems to cause some mailers a bit of confusion: email bounces. What’s the difference between a hard bounce vs. soft bounce?

At SendGrid, we probably position soft bounces a little differently. We look at soft bounces more as blocks—or short-term issues. These blocks shouldn’t be added to a suppression list. But, hard bounces (invalid email addresses and non deliverables) should be. Read on to learn about:

  • What is an email bounce
  • Hard bounce vs. soft bounce
  • Reasons why emails bounce
  • How to improve your email bounce rate

What is an email bounce? 

An email bounce signifies the non-delivery of your email message. When this happens, the mailer will receive an automatic notification of the delivery failure. This failure originates from the recipient’s mail server for a number of reasons (explained below).

Contents of a bounce message

Usually, the bounce message will give you important information to help you identify the reason for the email delivery failure. This includes the following:

  • The time and date the message bounced
  • The mail server that bounced it
  • The RFC code and reason for the bounce (according to the RFC, hard bounces are depicted by a 5XX code and soft bounces by a 4XX code. However, not all ISPs adhere to that code consistently, so there could be exceptions to this rule)

Hard bounce vs. soft bounce

At SendGrid, there are two types of bounces that you can receive—a soft bounce/block or a hard bounce. Think of soft bounces as blocks that are a short-term issue—you don’t need to permanently take these addresses off of your list. However, hard bounces are either invalid or non-existent addresses that should be removed immediately.

Soft bounce

A soft bounce means that the email address was valid and the email message reached the recipient’s mail server. However, common reasons it bounced back include:

  • The mailbox was full (the user is over their quota)
  • The server was down
  • The message was too large for the recipient’s inbox

At SendGrid, we continue to attempt to send these messages for up to 72 hours until the message is delivered. If a message is continuously deferred for 72 hours, we convert these addresses to a block/deferral list.  (A deferral list is not a suppression list.)

Hard bounce

A hard bounce occurs when the message has been permanently rejected either because:

  • The email address is  invalid
  • The email addresses doesn’t exist

At SendGrid, we add these hard bounced addresses to a suppression list. What this means is that even if you send a message through us for that user, we will not even try to deliver to that address, because we know it’s no longer good. Continuing to try to send to a known bad address will harm your reputation with the receiver, so we prevent that.

How to improve your bounce rate 

The best way to reduce the number of bounces is by following some key email deliverability best practices. This includes the following:

  • Clean your lists: Periodically remove inactive subscribers, invalid emails, and non-responders from your list. Use automated tools to identify and purge these addresses.
  • Segment your lists: Categorize subscribers based on activity levels and engagement. This helps in tailoring content that is more likely to be welcomed (and less likely to be ignored).
  • Manage bounces actively: Uses features in your email platform, such as SendGrid's bounce list management, to automatically remove addresses that consistently cause hard bounces. Set rules for how many times you will attempt to resend emails that soft bounce before considering removal.
  • Confirmation emails: Always send a confirmation email that requires new subscribers to verify their email address. This extra step helps prevent typographical errors and confirms the subscriber's interest.
  • Welcome emails: Following the confirmation, send a welcome email as part of the initial engagement. This sets the tone for the relationship and can improve long-term engagement and deliverability.
  • Regularly monitor: Keep a close watch on your email delivery and bounce rates. Analyze patterns over time to identify when your campaigns are suffering from deliverability issues.
  • Respond to changes: If you notice a sudden increase in bounce rates, investigate and fix potential causes such as issues with your email server or changes in email content that might be triggering spam filters.

Achieving high email delivery rates can be a challenge, but not if you know the ropes. For more tips, download our best practices guide: Tips and Tricks to Stay Out of the Spam Folder.

Frequently asked questions about email bounces

1. What is the difference between a hard bounce and a soft bounce in email?

A hard bounce indicates that an email has been permanently rejected either because the recipient's email address is invalid or does not exist. On the other hand, a soft bounce signifies a temporary delivery issue, such as a full inbox or a server temporarily unavailable. Hard bounces should be removed from your mailing list immediately to maintain a healthy sender reputation, whereas soft bounces can generally be retried within a short period.

2. How can I identify a soft bounce from an email error report?

A soft bounce is typically indicated by a 4XX SMTP error code in your email bounce reports. Common reasons for a soft bounce include a full mailbox, large message size, or the recipient's server being temporarily down. The bounce message will often provide the specific reason and the error code.

3. What does a soft bounce mean for my email campaign’s deliverability?

A soft bounce in your email campaign might temporarily impact your deliverability. It suggests a short-term issue with the recipient’s email server or mailbox. Monitoring the frequency and reasons for soft bounces can help you take proactive measures, such as resizing emails or rescheduling sends, to improve deliverability and guarantee your communications reach the recipient (eventually).

4. How do I reduce the rate of soft bounces in my email marketing efforts?

To reduce soft bounces, keep your email sizes within acceptable limits and clean up your maling lists—you want to avoid sending to full inboxes or temporarily unavailable servers. Regularly clean your email lists and segment them based on engagement.

Boost your email delivery rates

Want to lower your email bounces and bump up your deliverability rates? Sign up for a free Twilio SendGrid account and let us help you make it happen. We can empower your email program with email list hygiene tools, double opt-in features, delivery analytics, and more.

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