First, let’s define what we mean when we say an email has “bounced”: an email that was sent was never received in the recipient’s inbox, and an error message was returned to the sender by the mail service (or mail server) at the recipient’s location. This is different from email that disappears, since the sender at least receives some indication that the message failed.
When someone sends an e-mail, they are telling their e-mail service to look for the mail server for the domain of the recipient (e.g. blacklistmonitoring.com), and send the message to it. Once the e-mail is received by the recipient's mail server, the mail server looks at the message to determine if it will let the message pass through the server to the recipient’s email account. If the recipient's mail server isn’t accepting e-mails from the sender's address (e.g. it has blocked or blacklisted the address), the server will reject the message and it will consequently bounce back to the sender. The message will also bounce back to the sender if the mail server on the recipient's end is busy and cannot handle the request within a reasonable amount of time (usually 24-72 hours).
If an e-mail is returned to the sender before being accepted by the recipient's mail service, the resulting bounce is often called a “hard bounce”. Think of it like this: the recipient’s email account didn’t even get a chance to accept or reject the email – it was bounced from the server as though it hit a hard wall.
If an e-mail has been accepted by a recipient's mail service, the message can still bounce. The mail service has to determine if the recipient (for example, firstname.lastname@example.org) actually exists and if that account is allowed and willing to accept e-mails from the sender. If the recipient's stated email address does not exist on the mail server, then the message will be rejected because there is no one to deliver it to, unless the mail server has a “catchall” address specified. 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. If the message exceeds the maximum message size specified on the email server, then the message will bounce if it exceeds that size. When an e-mail is returned to the sender after it has already been accepted by the recipient's email server, it is called a “soft bounce”.
Occasionally, a network failure or busy mail server at the sender or recipient end will cause an e-mail to bounce back to the sender. Typically, a bounced e-mail returns to the sender with an explanation of why the message bounced. If the recipient has a backup mail server, the email may sit in a queue until the problem is corrected.
A less common type of error results from infinite forwarding, i.e. the original recipient account forwards a message to an account that either directly or indirectly forwards the message right back to the original recipient account.
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, orQuota 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. "blacklistmonitoring.com", 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. Read the other articles on this site for more information on blacklisting.
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