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Why do emails bounce back?

Lets define some terms:

  • A "Bounce" is where the proverbial brick wall meets the email. This happens when a recipient's mail server rejects your email message.
  • A "Soft Bounce" is often a temporary problem, thus the descriptive word "soft." It happens when the mail server confirms the recipient's email address, but even so, cannot deliver the message. The recipient's mailbox may be full or inactive, the recipient's mail server may be temporarily down or the connection may have been broken.
  • A "Hard Bounce" is a message that's permanently undeliverable because the address is non-existent or invalid, or because the recipient's mail server is blocking your mail server.

When a user attempts to send an e-mail, he is telling his e-mail system to look for the domain of the recipient (for example, and the domain's mail server. Once the e-mail system makes contact with the recipient's mail server, the mail server looks at the message to determine if it will let the message pass through the server. If the recipient's server has predetermined that it is not accepting e-mails from the sender's address (for example, if it has blocked the address for anti-spamming purposes), the server will reject the message and it will subsequently bounce back to the sender. The message will also bounce back to the server if the mail server on the recipient's end is busy and cannot handle the request at that time. When an e-mail is returned to the sender without being accepted by the recipient's mail server, this is called a hard bounce.

Once the e-mail has been accepted by the recipient's mail server there are still ways for the message to be rejected. The mail server has to determine if the recipient (for example, actually exists within its system and if that recipient is allowed to accept e-mails. If the recipient's address does not exist on the mail server, then the message will be rejected because there is no one to deliver the message to. If the sender misspells the recipient's address (for example, then the system will recognize this as a nonexistent address and bounce the message back. If the recipient exists but does not have enough disk space to accept the message (i.e., if his e-mail application is filled to storage capacity) then the message will bounce back to the sender. Some mail systems predetermine a maximum message size that it will accept and will automatically bounce the message if it exceeds that size and some mail systems predetermine a maximum amount of disk space the user is allowed to occupy on the server. When an e-mail is returned to the sender after it has already been accepted by the recipient's mail server, this is called a soft bounce. Some mail servers are programmed to accept incoming e-mails and store them for further analysis without initially checking to determine if the recipient exists or is even capable of receiving the message.

According to a recent Association for Interactive Marketing (AIM) survey, 77% of respondents had bounce rates up to 10%, and 23% had rates greater than 10%.

Why so many bounces and what can you do about them?

Email address churn in your house list
People change ISPs, jobs and email addresses at random. Often you'll be the last to know.

The use of free email accounts
Many people who use free email accounts do so as a secondary mailbox. As a result, they do not check their mailbox often. Free email accounts, and some paid accounts, can hold only a limited amount of email, so many times newsletters and advertising email will bounce back as undeliverable.

Spam filters and blocking
ISPs and corporations are paying close attention to incoming email in the effort to block spam, or unsolicited email. Anti-spam filters scan email from and subject lines as well as email body copy for certain language. They can also detect mailing patterns, frequency and volume. Your legitimate, permission-based email could be bounced back to you by a spam filter, or your mail server might be flagged as a potential spam source. In either case, your messages won't make it through.

What can you do about this

To maintain the health of your mailing list, you need to delete those email addresses that generate bouncebacks.  Cleaning up your mailing lists is also a good way to maintain a high-deliverability rate. You may need to contact your prospects by phone or snail mail to verify their email details.

You can review the reasons for emails bouncing back by referring to the bounceback message which has been sent to you. The bounceback reason code will be stored within the body of the email, or in an attachment which is generally named “details.txt”.

Bounce messages can vary in terminology. Some are easy to understand, and others more difficult. Some of the more common error messages follow.

MAILBOX NOT FOUND, Invalid mailbox, USER UNKNOWN: This error is quite common and should be obvious. Either the recipient doesn’t exist, or you’ve misspelled the address. Please check the spelling before contacting your ISP, etc.

Mailbox full, or Quota Exceeded: Your recipient has used up their space quota. Take them off your mailing list, ignore the bounce if they are on vacation, or use another address or method to contact them.

Mailbox unavailable: This could be another unknown user situation, or a temporary problem. The answer will become known after you try emailing the person a second time after a reasonable amount of time.

Host or Domain unknown: this means that the recipient mail server you're attempting to use, e.g. "", doesn't exist.

Unable to Relay: Mail is sent by relaying email from one server to the next. If a mail server doesn’t "know" the sender of an email or its recipient, the recipient’s mail server may bounce the email. Mail servers that do not enforce this requirement are called "open relays" and are frequently blacklisted, as they are easily used by spammers.

What constitutes a mail server not “knowing” a sender or recipient?

  • The "From" address might not match any account on the originating email server.
  • The Internet Service Provider (ISP) might not allow mail originating from another provider.
  • Your ISP might require you to authenticate before sending email – in this case a simple setting change in your email client should correct the problem.
  • The originating or receiving mail server might be misconfigured.

Content Filters: Content filters look for particular formats, words, phrases or other information in email messages to determine what is and is not, spam. Some providers use automated replies to messages that are determined to be spam – note that these automated replies also meet the definition of spam, so we don’t advocate their use.

Blacklist Filters: If you see a message that indicate your email was blocked, it was probably intentionally blocked because the receiving system thinks your ISP's mail server is a source of spam.  We have reviewed the public blacklists and have not found that our mail servers have been added to the list.

False Bounces: At times, you may receive a bounce message caused by viruses or spoof spammers (spammers that use other people’s addresses for the reply-to address). In this case, you can quickly determine if it’s your message that bounced.

Email Delivery Codes: Bounced email messages should include information on the error type. Sometimes the only error information is an error delivery code. Delivery codes are composed of three digits (X.X.X). The first digit gives the status of the email message, and the second, third digits give more detail:
2: the email was successfully sent;
4: there was a temporary problem while sending the email;
5: there is a permanent/fatal error related to the email

Following is a list of specific email delivery error codes, based on the Extended SMTP (ESMTP) standards, where X can be 4 or 5, depending on the error status type:

X.1.0 Other address status
X.1.1 Bad destination mailbox address
X.2.0 Bad destination system address
X.1.3 Bad destination mailbox address syntax
X.1.4 Destination mailbox address ambiguous
X.1.5 Destination mailbox address valid
X.1.6 Mailbox has moved
X.1.7 Bad sender's mailbox address syntax
X.1.8 Bad sender's system address
X.2.0 Other or undefined mailbox status
X.2.1 Mailbox disabled, not accepting messages
X.2.2 Mailbox full
X.2.3 Message length exceeds administrative limit.
X.2.4 Mailing list expansion problem
X.3.0 Other or undefined mail system status
X.3.1 Mail system full
X.3.2 System not accepting network messages
X.3.3 System not capable of selected features
X.3.4 Message too big for system
X.4.0 Other or undefined network or routing status
X.4.1 No answer from host
X.4.2 Bad connection
X.4.3 Routing server failure
X.4.4 Unable to route
X.4.5 Network congestion
X.4.6 Routing loop detected
X.4.7 Delivery time expired
X.5.0 Other or undefined protocol status
X.5.1 Invalid command
X.5.2 Syntax error
X.5.3 Too many recipients
X.5.4 Invalid command arguments
X.5.5 Wrong protocol version
X.6.0 Other or undefined media error

There are online tools that are available which allow you to check whether an email address is valid.  One such tool can be found at - simply enter in the email address and click on 'verify' and it will check to see whether the email exists.

This will not always be 100% accurate, as some email providers block the use of these validation tools - some government email addresses will always be returned as invalid, even when the email exists - but this will give you a fairly good indication of whether an email address if valid.

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