Soft Bounce Versus Hard Bounce: Email Delivery Issues Explained

What is a hard bounce?

A hard bounce occurs when the email address fails for a permanent reason.  The most common error is that the recipient’s mailbox does not exist.  Either the address was mistyped, or the address no longer exists.

If an email address in your list results in a hard bounce, it should be removed from your email list.  When judging sender reputation, hard bounces indicate that the list is of low quality, and might be spammy.  If the reputation score is low enough the ISP may limit your email deliverability. If you perform any email list hygiene - your first step is to remove all email addresses that hard bounce.

What is a soft bounce?

A soft bounce occurs when the email address does exist, but the message cannot be delivered due to a temporary problem.  The most common reasons for a soft bounce are a full inbox, a very large message, or the recipient’s email server being down.

A soft bounce is not graded as harshly by Internet Service providers, as the email address does exist. When an email soft bounces, it is recommended to try again after a period of time to see if the email account can receive messages.  When a soft bounce occurs due to a full inbox set the email to resend after 24 hours, and see if the message continues to soft bounce.  If a soft bounce continues for several messages, consider reaching out to your customer for a new address, or removing the email from your list.

If you receive a message that your email is too large, ensure that the images and attachments are optimized for email. It is a best practice to keep your marketing emails to a reasonable size in KB.

Proactively reducing e-mail bounces

In the last section we discussed what to do for hard and soft bounces after the fact, but how do you prevent them in the first place?  Wouldn’t it be better to prevent email bounces in the first place?

It is impossible to prevent all bounces, inboxes fill up, and employees leave their job.  Bounces come with the territory of sending email marketing.  But we can take steps to prevent bad data from entering our list in the first place.

Opt-in

If your customers sign up to your email list, using a double opt-in approach is highly effective.  When your customer signs up, they must click a link in an email to confirm their subscription. In this way, you have confirmed that their address is valid - they have already replied to your email!

Email Validation APIs

If your organization acquires a new list of customer email addresses, you may have no idea how they were sourced, or if they are valid.  Your email correspondence is too crucial to the company to risk adding unverified addresses to the list.  Your chance for the double opt-in has passed, but we can still work to verify a list of addresses.

One way to verify email addresses is by using an email validation API.  These APIs take an email address as input.  The API tests several criteria about the address for validity.  Addresses that are rated as undeliverable can be pruned from the list, and you know you have protected your email campaign’s sender reputation!

One such API is available from AbstractAPI.  AbstractAPI’s email validation API is an easy way to check the validity of email addresses.  Sign up, and you’ll receive a free API key.  

You can check an address with the following URL (replace <apikey> with your API key):

https://emailvalidation.abstractapi.com/v1/?api_key=<apikey>&email=email@example.com

This will provide the result:

{"email": "email@example.com",

"autocorrect": "",

"deliverability": "UNDELIVERABLE",

"quality_score": "0.00",

"is_valid_format": {"value": true,"text": "TRUE"},

"is_free_email": {"value": false,"text": "FALSE"},

"is_disposable_email": {"value": true,"text": "TRUE"},

"is_role_email": {"value": true,"text": "TRUE"},

"is_catchall_email": {"value": false,"text": "FALSE"},

"is_mx_found": {"value": false,"text": "FALSE"},

"is_smtp_valid": {"value": false,"text": "FALSE"}}

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The address email@example.com fails as an invalid address.  Reading through the rest of the JSON - we can determine common reasons an email address would fail. Let’s look at the parameters that each email address is graded against:

1. Invalid format:  If there is no “@” in the address, or at least one dot after the @.
2. Disposable email: An email address that “self-destructs” after a period of time. These addresses will stop working after a period of time and become a hard bounce.
3. Free email: providers like Gmail, and Yahoo mail - free email services can be used for fake addresses (but also are used by millions every day!)
4. Role email: These are not addressed to a specific person but to a team (support@, billing@, helpdesk@, etc.).
5. Catchall emails: A “catchall” email address is used to prevent bounces - all failed messages just land in an inbox for an email administrator to reroute (or just delete).
6. MX: The DNS MX record directs email to the mail server. If there is no MX server, the email may not be deliverable.
7. SMTP:  Simple Mail Transfer Protocol - a way to send and route messages.

In the case of our email address, there is no MX or SMTP server at example.com. All emails sent to this domain will fail.

If we try again with the Gmail address “douglasssillars@gmail.com”, it also returns as undeliverable:

{"email": "douglasssillars@gmail.com",

"autocorrect": "","

deliverability": "UNDELIVERABLE",

"quality_score": "0.00",

"is_valid_format": {"value": true,"text": "TRUE"},

"is_free_email": {"value": true,"text": "TRUE"},

"is_disposable_email": {"value": false,"text": "FALSE"},

"is_role_email": {"value": false,"text": "FALSE"},

"is_catchall_email": {"value": false,"text": "FALSE"},

"is_mx_found": {"value": true,"text": "TRUE"},

"is_smtp_valid": {"value": false,"text": "FALSE"}}

In this case, there is an MX server, and the email format is valid - but the address just does not exist.  Running your new, untested email list against this API will allow you to filter out the hard bounces before you send a message - protecting your sender reputation score.

FAQ

Is an out-of-office message a bounce?

Generally, no. An Out-of-office message (OOO) is successfully delivered - it just won’t be read for a certain amount of time.  This may affect your “open rate” as the receiver won’t open your message until they return to the office. So, while not a ‘soft bounce’ specifically, many email marketing tools will also report the number of OOO messages.

How does a high bounce rate affect my email marketing analytics?

‍A high bounce rate can skew your email marketing analytics, leading to inaccurate engagement metrics. It affects open rates, click-through rates, and overall campaign performance analysis. By inflating the number of undeliverable emails, it makes it harder to gauge the true interest and engagement of your audience.

Can changing my email service provider (ESP) reduce my bounce rates?

‍Switching ESPs might temporarily impact your bounce rates due to differences in delivery infrastructure and IP reputation management. However, reducing bounce rates fundamentally requires addressing the quality of your email list and sending practices. A new ESP may offer better tools for managing bounces and improving deliverability, but it's not a standalone solution.

Are there any industry standards for acceptable bounce rates in email marketing?

‍Yes, industry standards suggest that a healthy email campaign should aim for a bounce rate lower than 2%. Rates above this threshold indicate issues with list quality or sending practices. However, acceptable bounce rates can vary slightly depending on the industry and the type of audience you are targeting. Regular monitoring and maintenance of your email list are crucial for staying within these standards.