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Understanding Bounced Emails

What causes Emails to Bounce and How to Prevent Bounces

When an email is rejected by your customer’s email server, it’s called a bounce. All this means is the email was not delivered to the recipient's inbox. When this happens, you, as the sender, will receive a Non-Delivery Report (NDR) in your inbox. 

A Non-Delivery Report (NDR), also known as a Bounce Message, is a report that is automatically generated by a mail server to inform you, as the sender, that your email message was not successfully delivered. The NDR comes in the form of an email from the sender's mail server and contains information on why the delivery was unsuccessful.

NDRs are generated by mail servers based on a standard SMTP 500 error coding system.  These errors can occur at any stage during the mail delivery process. 

Every failed email scenario will have its own SMTP 500 code that informs the sender why the email has ‘bounced.’


Type of Bounces

There are a variety of reasons why emails bounce. Understanding the causes of why your emails bounce is important to maintain a healthy email marketing campaign. High bounce rates can have a negative impact on your email delivery rate for future campaigns. 

A Non-Delivery Report (NDR) will have three main components: 

  • The date and time the email bounced
  • The mail server that bounced it
  • The reason why your email bounces

The most information component of an NDR or bounce message will be bullet point #3, ‘the reason why your email bounced’

Different email servers all use different “response codes” to let you know why the email address bounced. To help simplify these reasons, we can categorize the reasons into two types;  “Hard Bounces” and “Soft Bounces.” 

Hard bounces are permanent email delivery failures. There are several categories of a Hard Bounce:

Below are some of these Hard Bounce categories"

1. The ‘Recipient’ does not exist: 

Pretty straightforward - It’s like the post office saying there’s no one with that name in the building. 

How to Fix:

  • Check the email spelling to ensure there are no typos
  • Check email validity - learn more about how to read RocketReach data here (icon KB)
  • If the errors are fixed and the address continues to bounce, remove it from your lists

2. The Recipient’s mailbox is full: 

This happens when inboxes reach their storage capacity because they’re not being checked.  This may happen if companies do not disable/forward former employees' email addresses and the email becomes abandoned.  

How to Fix

  • If you have access to phone data for the contact, we recommend reaching out to your customer by phone and letting them know their mailbox is full.
  • Or try contacting the customer by phone to see if they have a new email address.

Try one of our +Phone plans to see if we have a valid phone number for the contact


3. Blocked Email Address

This reason is not so straightforward.  Emails are considered blocked if the inbox may exist but the Recipients email server has decided not to deliver the email to the recipient's inbox 

Email servers have all sorts of security policies that your “sent from” email address needs to pass before it accepts a message and will take into consideration your sender's reputation.  Many companies have strict security policies set on their email servers to protect their employees and their company from bad actors.’ 

Email addresses can be blocked if the server perceived the email to be spam. The Recipient's email server might be concerned about something in the email like a link or an attachment or concerned with the sender being a large sender, an email that sends several emails. 

Blocked Email reasons will include DMARC; DKIM, and SPF authentication failures.

How to Fix: 

  • If you’re getting a blocked bounce from a smaller domain, you may be able to resolve the issue yourself by contacting the email server of the recipient directly. 
  • If you’re getting blocked bounces from a larger domain (Yahoo, Gmail, etc.) you will need to contact your Email Service Provider (ESP) for assistance.


4. Rejected by Spam Filters

It is possible that there is some content in your email or the subject line that is deemed as “spam” by the recipient's email server. 

And sometimes, it may have nothing to do with the email you sent!  Some email servers manage email based on the behavior of their customers. For example, if a customer never opens your emails, the next one you send could be filtered automatically into the spam folder.

Another similar scenario is If a customer has marked your email as spam in the past, all your future emails will likely be treated as spam going forward.

How to Fix

  • Suppose you have your email connected to our Compose tool and notice a customer is frequently listed as “did not open” in your reporting. In that case, it may be time to rethink how often you send them emails, figure out what content truly engages them, or remove them from your list altogether.
  • Use placeholders in your Email drafts to auto-populate the contact's name.  Spam filters are more likely to flag your email if it’s addressed to your recipient's email address and not their name.
  • Use a verified business email - Anonymous and free email domains, such as Gmail, are often filtered more aggressively than verified email domains
  • Reach out to your Email Service Provider to see if they have any additional tools to help prevent Spam Blockers.


5. Soft Bounces

When your emails soft bounce, it means they haven’t reached your recipient’s inbox – but they might… eventually! Think of these emails as being somewhere in limbo. Some may make it to the mailbox, others will end up hard bouncing.

Here are some of the reasons your email could soft bounce:

  • Your file may have been too large.
  • Possible server outages on either end
  • The recipient’s email has been suspended.
  • The recipient email server has been receiving too many emails for a period of time.
  • Other technical issues



Now that you know what hard and soft bounces are, you can view bounce details in the Non-Delivery Report (NDR). 

It's a good idea to keep a close eye on your bounce rate to be sure your emails reach your contacts. This can help you to abide by spam laws and avoid bounce suspensions.

Have more questions? Talk to us
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