Marketing Automation

Email Bounces and Error Codes

What Do The Different Bounce Reasons Mean

There are multiple bounce types that you could see in the report for a sent email. All of the bounce messages we receive are sorted into soft bounce categories or, if it’s a permanent delivery failure, it’s classified as a hard bounce.

Bounce Category Definitions

The following categories will not apply to every bounce type out there. We take a simplified approach to bounce message interpretation based on best practices.

General bounce

This is treated as a soft bounce because we cannot determine the exact reason for delivery failure. Typically, this bounce type is associated with a technical issue such as, “Connection timed out,” but we will also classify a bounce as “general” if the response from the recipient server is open to more than one interpretation. It could be a non-standard error message, or too vague to be useful.

DNS failure

The recipient’s email server is currently unable to deliver your email due to DNS issues on their end. This may or may not be a temporary problem. The error could be due to the mail server being down, or there was a typo when it was set up, or maybe the destination domain doesn’t exist.

All we know is that the DNS host is unreachable, therefore we treat this as a soft bounce to allow some time for the problem to be rectified.

Mailbox Full

The email server cannot deliver your email because the recipient’s inbox is full. Most email applications have a set amount of storage an individual user can use for email. If this quota is exceeded the server will not let any more mail through, but it will also usually alert the mailbox owner so they can do something about it.

While it may be the case that your recipient hasn’t put aside time to make some space, it could also be a 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 for a house.

We treat these as a soft bounce in case it’s temporary, but if the issue continues we’ll convert it to a hard bounce for you.

Message Too Large

The size of your email — including all headers, text and images — is larger than the maximum size the recipient’s mailbox allows. The bounce message returned doesn’t include information on what the size limit is, but we advise you do not send messages bigger than 99Kb.

Mail Block - Relay Denied

Your email has bounced due to a temporary error, which could be on the sending or receiving side. “Relay” simply refers to the transmission of your email from our server to the receiving server, which has most likely been denied due to user error. This type of bounce usually occurs when the sender’s message is not authenticated, but it can also be due to a misconfigured server on the recipient side.

Mail Block - General

The recipient’s email server is blocking inbound mail from our server, which may be due to a blacklisting. A mail block is recorded when the receiving server blocks an email completely; rejecting the message without any attempt to deliver it to the inbox.

The most likely reasons for this block are:

  • Your reply-to address is blacklisted.
  • One of our sending IPs is temporarily blocked.
  • One of our sending domains is temporarily blacklisted.
  • The receiving server only accepts whitelisted senders.
NOTE: You should only try resending this email if the bounce is due to our IP or sending domain being blocked or blacklisted. Please contact us to find out.

Mail Block - Spam Detected

The recipient’s email server has blocked your email on the basis that the content resembles spam. This mail block is often triggered by something detected in your email content but can also be your reply-to address or brand name that has a poor reputation. We treat this as a soft bounce due to the fact that some mail servers and email providers respond with false or incorrect error codes.

Transient Bounce

The recipient mail server can’t deliver your email but will keep trying for a limited period of time. We treat this as a soft bounce, as when the recipient mail server retries the message could be delivered.


The recipient has installed software as an anti-spam measure, that only accepts email from previously authorized senders. If the software doesn’t know the sender, a challenge email is returned, requiring a specific action before the original email will be sent to the user.

Bounce - but no email address returned

The recipient mail server bounced your email but did not indicate which address it was bouncing on behalf of. We’ve determined the recipient based on the content of the bounce.

Hard bounce

Your email is permanently undeliverable to this email address. The address is either fake, was entered incorrectly, or the user mailbox or domain is no longer active.

For example, the following are all hard bounce error messages:

  • Unknown or illegal alias
  • Address rejected
  • No such user here
  • Bad destination mailbox address
  • Rule imposed mailbox access for refused: user invalid
  • Standard set of SMTP error codes

Traditional Bounce Codes

Code Explanation
421 (domain) Service not available, closing transmission channel
450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451 Requested action aborted: error in processing
452 Requested action not taken: insufficient system storage
500 The server could not recognize the command due to a syntax error.
501 A syntax error was encountered in command arguments.
502 This command is not implemented.
503 The server has encountered a bad sequence of commands.
504 A command parameter is not implemented.
550 User’s mailbox was unavailable (such as not found)
551 The recipient is not local to the server.
552 The action was aborted due to exceeded storage allocation.
553 The command was aborted because the mailbox name is invalid.
554 The transaction failed for some unstated reason.

Enhanced Bounce Codes

Code Explanation
5.0.0 Unknown issue
5.1.0 Other address status
5.1.1 Bad destination mailbox address
5.1.2 Bad destination system address
5.1.3 Bad destination mailbox address syntax
5.1.4 Destination mailbox address ambiguous
5.1.5 Destination mailbox address valid
5.1.6 Mailbox has moved
5.1.7 Bad sender’s mailbox address syntax
5.1.8 Bad sender’s system address
5.2.0 Other or undefined mailbox status
5.2.1 Mailbox disabled, not accepting messages
5.2.2 Mailbox full
5.2.3 Message length exceeds administrative limit
5.2.4 Mailing list expansion problem
5.3.0 Other or undefined mail system status
5.3.1 Mail system full
5.3.2 System not accepting network messages
5.3.3 System not capable of selected features
5.3.4 Message too big for system
5.4.0 Other or undefined network or routing status
5.4.1 No answer from host
5.4.2 Bad connection
5.4.3 Routing server failure
5.4.4 Unable to route
5.4.5 Network congestion
5.4.6 Routing loop detected
5.4.7 Delivery time expired
5.5.0 Other or undefined protocol status
5.5.1 Invalid command
5.5.2 Syntax error
5.5.3 Too many recipients
5.5.4 Invalid command arguments
5.5.5 Wrong protocol version
5.6.0 Other or undefined media error
5.6.1 Media not supported
5.6.2 Conversion required and prohibited
5.6.3 Conversion required but not supported
5.6.4 Conversion with loss performed
5.6.5 Conversion failed
5.7.0 Other or undefined security status
5.7.1 Delivery not authorized, message refused
5.7.2 Mailing list expansion prohibited
5.7.3 Security conversion required but not possible
5.7.4 Security features not supported
5.7.5 Cryptographic failure
5.7.6 Cryptographic algorithm not supported
5.7.7 Message integrity failure

*Mail server administrators can customize messages that accompany bounce codes. Resource: RFC2821 (April 2001), RFC3463 (January 2003); SMTP Enhanced Codes Registry (IANA, February 2012); What those SMTP Error Codes Mean and Why You Should Care (Getting Email Delivered, January 2010)

Note: If you need to know more about a specific bounce in your report, please contact us. We can check our bounce logs for the exact DSN returned and investigate further.