Email bounces happen from time to time, but high bounce rates can put your sender reputation in jeopardy, restricting the number of emails you can send to your audiences.
Further harming email deliverability is the possibility of being blacklisted if you have many hard bounces. Once you’re on email blacklists, you will have to fight tooth-and-nail to get off ’em. It’s doable but difficult.
Familiarizing yourself with email bounce codes will go a long way toward helping you understand why your messages bounce so you can reduce your bounce rates.
This is a detailed article, but it’ll still take less than 10 minutes to read. In this little guide, we will explain how to manage email bounce codes, and share a full list of email bounce codes and what they stand for.
Table of Contents
What Is an Email Bounce?
Let’s take it from the top and briefly explain what it means when an email bounces. A bounced email is any email that has not reached the recipient. The internet service provider or ISP returns it to you, which will happen instantly after you send it.
Depending on the cause of the bounce, you can try again later, recheck the recipient’s details, and send again immediately, or contact a different email address.
Bounces can be categorized in two ways: hard or soft. Let’s review the differences.
Soft Bounce vs Hard Bounce
A soft email bounce is often a temporary error caused by the recipient or their server, not the sender. Here are some reasons you might get a soft bounce when you email a segment of your contact list:
- Their email server can’t relay the message from your email server.
- The recipient’s email server requirements aren’t met.
- The email doesn’t pass the antivirus of the recipient server.
- The email got caught in the recipient’s spam filter.
- The email didn’t pass the recipient server’s authentication requirements, such as DMARC.
- The message violates the recipient server’s email policies.
- The recipient’s server blocks the content in the email.
- The recipient’s domain name no longer exists.
- The email is too big, usually due to attachments.
- The recipient server cannot receive emails due to sending out too many.
- The recipient’s server is offline.
- The recipient’s server is down.
- The recipient’s mailbox is incorrectly configured.
- The recipient’s mailbox is full.
The recipient doesn’t receive an email that your message to them bounced, so it’s up to them to improve matters on their end (or their ISP to do it, depending on the source of the soft bounce).
The other type of email bounce is a hard bounce. If soft bounces are temporary, hard bounces are permanent. For example, the recipient’s server might have blocked your email address. The email address you’re trying to reach might no longer exist, or you’ve contacted an invalid domain name.
Unlike a soft bounce, which the email recipient or their ISP might ameliorate in due time, there’s usually no return from a hard bounce. You can check the recipient’s email address and confirm you have everything spelled correctly, but you must remove them from your list outside of that.
You can calculate your soft vs hard bounces through your email bounce rate, which is the number of emails bounced divided by how many you sent.
A good bounce rate is around one percent (1%).
Okay, So What’s an Email Bounce Code?
Every email bounce, whether soft or hard, generates an email bounce code.
The bounce code explains why your email didn’t reach the intended recipient. You can receive a regular bounce code or an enhanced SMTP bounce code, which can be very different.
Traditional and Enhanced SMTP Bounce Codes – What’s the Difference?
SMTP is short for Simple Mail Transfer Protocol. All email servers use an SMTP to relay, send, and receive emails. A traditional SMTP bounce code will have a three-digit identifier indicating whether the email was sent.
The three-digit code tells you precisely why the email failed to reach the recipient, provided you understand what the code means.
In 2003, enhanced SMTP bounce codes were born. Traditional SMTP codes were helpful but only revealed so much, while the enhanced codes shed much more light on the matter. An enhanced SMTP code also has three digits but periods in between them.
What does each of the three digits mean? The first represents the email delivery class. A 2.X.X refers to a delivery that reached the recipient. If you see a 4.X.X, that indicates a soft bounce or a temporary issue. A 5.X.X is a hard bounce due to a permanent issue.
To recap, the first digit of an enhanced SMTP code can be a 2, 4, or 5.
The second digit represents an error code. If you see a 1 in the second spot, your email didn’t send because of an address issue. A 2 represents a mailbox problem, and a 3 means it’s an issue with the mail server.
The second digit of an enhanced SMTP code can be a 1, 2, or 3. Then, there’s the third digit, which further elaborates on the email issue. This can be between 0 and 5.
ISPs use traditional and enhanced SMTP codes to help email senders understand why their messages failed.
Understanding Email Bounce Codes: The Full List and What They Mean
Let’s put all the information together from the prior section and review the common bounce codes and what they mean when you receive them.
Status Group Codes
A status group code refers to the first of the three digits in an enhanced SMTP code and is as follows.
|Status Group Code
|What It Means
|Informational group indicating the server received and is still processing your request.
|The server successfully got your request and has accepted it.
|Redirection group indicating that the server must do more to fulfill the request.
|Client error indicating the server could not fulfill the request, or it has bad syntax.
|Server error indicating the server couldn’t fulfill the request but not due to any errors on the sender’s part.
Soft bounce codes begin with 4, such as 4XX (a traditional SMTP bounce code) or 4.X.X (an enhanced SMTP bounce code). Here’s an overview of each code and its meaning.
|What It Means
|An error occurred when transferring the message, and the connection timed out.
|The recipient’s server cannot receive the message because of excess connections or messages. The email is deferred until the recipient can receive messages again.
|The recipient’s inbox is full and has no storage space left.
|The recipient has a file overload and is out of memory or disk space.
|The arriving mail queue was stopped due to the exchange server.
|The recipient’s server has stopped responding.
|The recipient’s server lost connection while the transmission occurred.
|The recipient has exceeded their email hop count and caused an internal loop error.
|The incoming server had an issue, causing the email you sent to time out.
|Routing issues, possibly caused by configuration trouble, prevented the server exchange.
|The mailbox is busy or otherwise unavailable.
|The message could not be processed and was aborted.
|The system lacks enough storage to send the email.
|A local anti-spam filter prevented the email from going through.
Hard bounce codes begin with a 5, either a 5XX or a 5.X.X. Here is a complete overview of all the codes you might receive when a permanent error occurs during email sending.
|What It Means
|The email address doesn’t exist.
|Command arguments encountered a syntax error.
|The server cannot implement the requested command.
|A sequence of commands has prevented the server from proceeding.
|The recipient could not implement the command parameter.
|You’re trying to reach a bad or invalid email address.
|You’re trying to reach a bad or invalid destination.
|The recipient’s server had a DNS error and could not find the domain name for the recipient.
|An address has been misspelled or cannot be authenticated.
|The recipient’s mailbox address is deemed ambiguous, and cannot receive the message.
|The mailbox address is invalid.
|The mailbox has moved.
|A bad mailbox address syntax has prevented the recipient’s server from receiving your message.
|A bad system address has prevented the recipient’s server from receiving your message.
|The recipient’s mailbox status is undefined.
|The recipient’s mailbox is no longer accepting messages due to being disabled.
|The recipient’s mailbox is full, and they can no longer receive messages until they empty it.
|You’re trying to send an email that exceeds the limits of the recipient’s server.
|The mailing list could not be expanded.
|The recipient’s email address is invalid, the recipient’s server blacklisted your email address, or authentication issues.
|The system cannot accept network messages.
|The system cannot accept some selected features.
|The size of the message exceeds the allowable system size.
|The routing status or network is undefined.
|An anti-spam filter or other measure rejected the message by recipient address.
|The routing server failed.
|The server cannot route.
|The network is congested.
|The network detected a routing loop.
|The period for delivery expired.
|The server could not find the recipient’s mailbox.
|The recipient isn’t a local part of the server.
|The storage allocation was exceeded, and the request was aborted.
|The mailbox name was invalid, and the request was aborted.
|The email transaction failed, although for reasons not stated.
|The protocol version is incorrect.
|A media error occurred.
|The type of media sent is not supported.
|The required conversion is prohibited.
|The required conversion is not supported.
|The conversion occurred but with a loss.
|The conversion could not occur.
|The security status is undefined.
|The message was refused due to unauthorized delivery.
|The server prohibited mailing list expansion.
|The required security conversion could not occur.
|The required security conversion is not supported.
|A cryptographic error or failure occurred.
|The required cryptographic algorithm isn’t supported.
|The message integrity failed.
How To Manage Email Bounce Codes
Bounced emails put your sender reputation at risk if they happen frequently enough, including hard and soft bounces alike.
Although you can’t control what your email recipients do or how their server behaves, you have plenty within your power to reduce your company’s email bounce rate.
Here are some pointers.
Pause emails to the recipient
You’ve determined the intended recipient cannot accept emails at current, likely due to a temporary issue. Check the bounce code; if you see a code beginning with a 4, it’s a soft bounce that should be rectified when possible.
In the interim, it does no good to continue sending emails, as they will all bounce. Instead, temporarily pause emails to that recipient. A pause might mean 24 or 48 hours or even a week. It’s your choice how long you decide to wait.
Try sending the email again and decide how to proceed based on whether you get the same error, a different error, or no error.
Remove inactive email addresses
If you’re still receiving 4XX or 4.X.X email bounces from the recipient, despite a temporary bounce, that doesn’t mean it will always be addressed. Soft bounces are on the part of the recipient, such as a full email inbox.
You don’t want to hold your breath waiting for the email recipient to empty their inbox, as you don’t have any way of knowing how long you’ll wait. Instead, remove them (and any other inactive addresses) from your email list.
If the recipient gets their email issues fixed and realizes they aren’t receiving your emails, they can sign up for your email list again. In the meantime, keeping them on your list could put you at risk of your ISP assuming you’re engaging with spam traps, thus hurting your sender reputation.
If you’ve received a 5XX or 5.X.X email bounce, remove the recipient immediately. These permanent issues are highly unlikely to be fixed.
Validate email addresses
Do you validate your emails before sending them? This is a great way to reduce the amount that will bounce. You can use free email verification through some services, so even startups can begin validating email addresses.
Use double opt-ins
People accidentally end up on email lists all the time, whether a company unscrupulously sold their email address as part of a list sale or they signed up and completely forgot.
Instituting double opt-ins during signup reduces the rate of unintended signups, no matter the reason. You’ll improve your open and click-through rates and reduce your bounce rate.
Follow good email hygiene
Double opt-ins are one part of proper email hygiene, which means following best practices to maintain your sender reputation.
Here is a list of behaviors to begin implementing if you aren’t already.
- Do not use spam words in your subject lines or email bodies.
- Don’t mislead or lie to your audience in the subject line.
- Make it simple for anyone to unsubscribe, so if they’ve changed their minds or no longer want to be part of your email list, they can very easily exit.
- Keep your email list full of engaged subscribers, removing inactive emails that bounce.
- Find an email cadence that works for your audience. Perhaps you email them once or twice a week, or maybe every few days, but it should be a rate most of your customers find agreeable.
- Optimize your emails for mobile and desktop users.
- Segment your audience.
- Personalize your emails.
- A/B test your emails before sending them.
- Ensure your hyperlinks and CTAs are clear about where they’ll redirect the user.
- Don’t use spammy practices in the preview text.
- Never attach anything to your emails unless the user already knows about it.
- Do not write overly long emails.
- Always send emails from the same server and address.
- Send welcome emails to all new subscribers, and without delay.
Watch your sending limits
Do you use a free email server? Many of them institute send limits.
For example, Gmail only allows paid users to send 2,000 emails daily, and it’s 500 if you have a free account. You can go through that number of emails fast as a large corporation, but all emails over the limit will bounce.
Consider moving to your own server with greater limits. This will also build trust and credibility. However, you can warm up your IP by gradually increasing your volume of emails before you launch an email blast.
If you don’t, your ISP will likely block your messages.
Email bounce codes are broadly categorized by soft and hard bounces. The codes reveal what went wrong so you can remedy the issue, even if that’s merely by pruning your email list.