Imagine you send a letter to an old  friend with whom you’ve lost contact.

It comes back after a week with a stamp that reads “undelivered.” Maybe your friend no longer lives at that address, or maybe the postman was unable to deliver the letter for another reason.

An email bounce is similar. When an email is rejected by your customer’s email server, it’s called a bounce. Basically, this means your email wasn’t delivered to the customer’s inbox. When this happens, you receive a Non-Delivery Report/Receipt (NDR) from your customer’s email server informing you about a delivery problem.

There are multiple reasons why emails bounce, and bounces can either be categorized as “Hard Bounces” or “Soft Bounces.” It’s easy to ignore the bounce section in your email reports, but if you do, they’ll sooner or later start to pile up.

Then you’ve got a problem.

Maintaining a clean list is essential to sending successful email marketing campaigns with a good response rate. If your account has a high email bounce rate it can have a negative impact on your delivery rate. There are some email service providers that allow you to track email deliverability and help you better understand how your emails are performing.

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Contents of a bounce message

Typically, a bounce message includes some identification cues as to why your email failed to deliver. This could include:

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

Take a look at the example below. I sent an email to “” and received a Mail Delivery Failure Notification with the error message, “domain name not found:”

This means does not exist, or Parker didn’t renew the domain name and it expired.

Address not found

Bounces can be equally frustrating for you and your customers. When your email bounces, it means you couldn’t reach out to your customers to tell them something, and in turn, your customers missed out on receiving important information from you.

Hard bounces vs. soft bounces

What is a hard bounce?

Hard bounces are permanent email delivery failures.

A hard bounce occurs when your customer’s email address is invalid or is no longer in use. Typically, the domain name (the bit after the @) no longer exists or it no longer has registered mail servers. But it could also be invalid due to typos, for example, “gnail” instead of “gmail.”

When an email bounces, the Internet Service Provider (ISP) sends back a message stating why your email bounced. Different ISPs and different mail servers all use different “response codes” to let you know why the email address bounced. There are several categories of a hard bounce:

1. Recipient does not exist

These bounces happen when your customer’s ISP says that an email address doesn’t exist. It’s like the post office saying there’s no one with that name in the building. If an email address continues to bounce over a long period of time, Constant Contact marks them as “Suspended,” and stops sending that customer emails for some time.

We do this because sending emails to the same non-existent email address can negatively impact the sending reputation for both your business and Constant Contact.

What you can do to fix this?

  1. Double-check these email addresses for obvious typos.
  2. Get in touch with your customer through another channel to see if they have a new email address.

After fixing the issues, try sending to them again.

  • If the errors are fixed and the address continues to bounce, remove it from your lists.
  • If you know the address is correct, it may be bouncing for a different reason, and you should contact our Support Team to troubleshoot.

2. Mailbox full

It’s likely you’ve seen this error message in the past. When an inbox is too full to continue receiving new email messages, the ISP sends back a “Mailbox Full” message. Usually this happens when mailboxes aren’t checked often enough and have reached their storage capacity.

So, while it may be the case that your customer hasn’t checked and cleaned out their email inbox in a long time, it could also be the sign of an abandoned mailbox. For example, someone sets up a free webmail account just for shopping-related emails, then stops signing in when they start saving money for a house.

What you can do to fix this?

  1. Reach out to your customer and let them know their mailbox is full.
  2. Get in touch with your customer to see if they have a new email address.

If the address keeps bouncing, you should remove it from your lists.

3. Blocked email address

When an email address is blocked, the ISP has decided not to deliver to it, possibly because it thought the email was spam. The ISP might be concerned about something in the email, or it rejects email from large senders.

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 reputation. If the email doesn’t pass one, but the ISP is not explicit about which policy, you are likely to see the bounce categorized here. This includes DMARC; DKIM, and SPF authentication failures.

What you can do to fix this?

  • If you’re getting a blocked bounce from a smaller domain, you may be able to resolve the issue yourself by contacting the ISP.
  • If you’re getting blocked bounces from a larger domain (Yahoo, Gmail, etc.) contact our Support Team to get it resolved.

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 an ISP. Some ISPs 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.

If a customer has marked your email as spam in the past, all your future emails are likely to be treated as spam going forward, even if the individual legitimately  signed up for your mailing list.

What you can do to fix this?

  • If you notice a customer is frequently listed as “did not open” in your reporting, it may be time to rethink how frequently you send them emails, figure out what content truly engages them, or remove them from your list altogether.
  • You have control over what you put into your email, so make sure to run a spam check before you hit send.

It is important that you pay attention to your bounce rate and your overall list hygiene. Check to see what type of automated protocols your email service provider has in place to facilitate the process.

Tip: Check out this two-minute video on how to remove bounced contacts from your list.

What is a soft bounce?

Soft bounces are temporary email delivery failures.

A soft bounce occurs when your email campaign goes as far as your customer’s mail server, which means that your email address was recognized, but the message is then bounced back as undelivered before reaching the customer’s inbox.

Soft bounces can occur when the recipient’s mailbox is full; the receiving server is down or swamped with messages; the message size is too large; the recipient’s settings don’t allow for an email from the sender; suspicious or spammy content has been detected; and many more reasons.

Below are the messages that result in a soft bounce:

1. Mailbox full

While typically categorized as a hard bounce, a mailbox full error can sometimes fall into the soft bounce category, at least initially. For example, you’ve been sending emails to a particular customer which have been delivered, and opened, in the past, but are now failing to be delivered. The ISP monitors the frequency of how often your emails fails delivery over two or three days, while they keep trying to send the email. During this period, the email is temporarily placed in the soft bounce category. Once the 2-3 day limit is up, if the email continues to bounce the ISP categorizes it as a hard bounce.

However, if this goes on for a longer period of time, it is highly likely that your customer’s ISP might start marking your emails as hard bounces. This usually happens when your customer hasn’t checked their inbox in a long time and their mailbox storage has reached its capacity.

What you can do to fix this?

  1. Reach out to your customer and let them know their mailbox is full.
  2. Get in touch with your customer to see if they have a new email address.

If the address keeps bouncing, you should remove it from your lists.

2. DNS failure

Domain name system (DNS) failure happens when your customer’s email server is unable to deliver your email due to DNS issues at their end. This may or may not be a temporary problem. The error could be due to the mail server being down, or a typo at set up, or a destination domain that doesn’t exist.

For example, DNS servers can sometimes crash or go offline. Perhaps the data center used by the service has been cut off from the Internet for some reason. Typically, these problems are resolved relatively quickly, but for some domains and servers, it may take a while longer.

What you can do to fix this?

  • Try sending your email after a day or two. If the failure is really temporary, it should be resolved in a short timeframe.
  • Try contacting your customer some other way in order to verify their address.

3. Challenge-response error

A challenge-response reply is a message sent by a filtering service installed by a customer to help filter emails from unknown senders and spam emails.

The filter automatically sends a reply with a “challenge,” usually a question or an action, and if the challenge is not completed properly or within a certain time frame, your email is not delivered.

What you can do to fix this?

  • Use permission-based email marketing. Run a double opt-in campaign where a customer must respond to an email confirmation in order to be added to your mailing list.
  • Spam-test your email campaigns before sending them out. Make sure you aren’t overusing words like “free” or “buy now,” or excessive punctuation–all major spam triggers.

4. Vacation/out of office/auto-reply emails

Auto-replies (such as an out of office notice) often indicate that your customer is temporarily unavailable. These notifications are useful when sending time-sensitive information to customers, as they alert you that your customer may not see your email until later.

What you can do to fix this?

These aren’t actual bounces, so you don’t have to do anything with these email addresses. These contacts will eventually see your email once they’re regularly checking their email again.

Note: You can potentially see vacation or auto-reply bounces from email addresses that aren’t contacts on your list if:

  • You send your email to a contact using a role address that is used by several different people, and one of them has an auto-reply enabled.
  • Your contact has forwarded your email to someone who has an auto-reply enabled.

5. Other errors

Sometimes an ISP doesn’t use a standard error message when they send back the bounce response code–we categorize these bounces as “Other.”

This is treated as a soft bounce because the exact reason for the bounce cannot be determined. Typically, this bounce type is associated with a technical issue, such as a timed-out connection.

What you can do to fix this?

  • Get in touch with the contact to see if they have a new email address.
  • If you know the address is correct, you should contact our Support Team to troubleshoot.

If the address keeps bouncing, you should remove it from your lists.

Tip: If a contact tells you they aren’t receiving your email and they aren’t on your bounce list, ask the contact to check their spam folder. If the email isn’t in the spam folder, you can have your contact safelist your “From Email” address in their email account or security program.

Bounce back to better email metrics

When either a hard or soft bounce occurs, it’s time to roll up your sleeves and take a long, hard look at your mailing lists. Bounces aren’t good for your sending reputation; they can have a negative impact on deliverability and the success of future email campaigns. Along with tracking all the other important email metrics, it’s a good idea to keep an eye on your bounce rates.

If addresses continually bounce, it may be time to take the next step:

  • Remove: take the contact off your list, but allow the contact to add themselves again later
  • Do Not Mail List: a removal, and the assurance  that the contact can never be added again

Use the type of bounce, hard vs. soft, to determine which action to take. Remember, lower bounce rates means your customers are hearing from you consistently, and rewarding your hard work with more opens, more clicks, and more business.

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