What is a Hard Bounce?

In email marketing, bounces refer to emails that the recipient is unable to receive, which results in an error (also known as a “bounce”) message from the intended recipient’s email server.

A hard bounce is a relatively permanent error, which means that repeated attempts to send the same email to the same email address will result in the same error message. Often, a hard bounce refers to domains or email addresses that do not exist.

By contrast, a soft bounce refers to a temporary error. If that error is fixed, usually by the recipient, then the email may be able to get through in the future. Examples of soft bounces include full mailboxes, mailboxes that have not been setup (although the email address itself is legitimate) and congested networks (preventing emails from getting through and to the intended recipient).

Hard bounce effect on deliverability

Hard bounces have traditionally been considered one of the factors used by spam filters and anti-spam software in determining whether messages from an email sender should be accepted. However, it is not considered as important as other factors, particularly those involved with engagement.

Hard bounces are a fact of email life. Many non-spam mistakes and events can produce hard bounces, including the following:

  • Misspellings or typographical errors
  • Syntax errors and mistakes
  • Email message size
  • Improper account setup
  • Wrong email provided by recipient
  • Connection issues

Nevertheless, it’s always a smart idea to conduct regular list hygiene to remove hard bounces from your emailing list. The more reliable email marketing platforms, from Responsys and Exact Target to Constant Contact and HubSpot, provide mechanisms for easily (sometimes automatically) removing email addresses that experience hard bounces.

STMP error codes

Both hard and soft bounces are characterized by error messages from the email server of the intended recipient. Most email programs today rely on the Simple Mail Transfer Protocol (SMTP) to transmit and receive email messages over the internet.

The following are the most common SMTP status codes (error notification) indicating a hard bounce:

 

SMTP Code SMTP Status SMTP Error Description/Explanation Recommended Steps
500 Syntax error: command not recognized 5.0.0 The most recent command from the sender’s server was not considered a valid SMTP command by the receiving server. Multiple 500 error messages from different receiving servers points to problems with the sending server, possibly with its firewall.
501 Syntax error: parameters or arguments not recognized 5.0.1 The receiving server receives the command as valid SMTP, but the receiving server does not recognize or accept the command parameters or arguments. Check the domain name to ensure that it’s active or the email address to ensure that it’s legitimate.
502 Command not implemented 5.0.2 The receiving server accepts the message from the receiving server, but is unable to execute the requested command. The receving or sending server may need to be updated.
503 Bad command sequence 5.0.3 The message “packets” were received by the receiving server in the wrong order. This is often due to some sort of network interruptions, such as from faulty routers or “noisy” lines/cables.
504 Parameter not implemented 5.0.4 The receiving server accepts the command and parameter but does not implement the parameter (or is awaiting additional parameters). The message may be missing a required authentication.
510 Bad target email address 5.1.0 The sender’s mail server has flagged the message as having an incorrect email address. Check the spelling or formatting (such as missing @) in the email address.
511 Bad target email address 5.1.1 The sender’s mail server has flagged the message as having an incorrect email address. Check the spelling or formatting (such as missing @) in the email address.
512 Domain name not recognized 5.1.2 A sending, routing or receiving server does not recognize the target domain name. Check the spelling or top level domain (.com, .org., .net, etc.) of the target domain.
513 Incorrect email address format 5.1.3 The sender’s mail server has flagged the message as having an incorrect email address. Check the format of the email address, such as the inclusion of quotation marks or a missing @ sign.
514 Bad target email address 5.1.4 The target email address is ambiguous. Check the spelling or formatting (such as missing @) in the email address.
515 Bad target email address 5.1.5 The target email address is invalid. Check the spelling or formatting (such as missing @) in the email address.
517 Bad sending email address 5.1.7 The sender’s email address has the wrong sytax. Check the spelling or top level domain (.com, .org., .net, etc.) of the domain in the sending email address.
518 Bad sending email address 5.1.8 The sender’s email address is invalid. Check the spelling or formatting (such as missing @) in the sending email address.
523 Message too large 5.2.3 The message exceeds the limits set by the recipient’s email server or platform. Reduce the email size or use a document sharing site (instead of attaching large files).
530 Authentication required 5.3.0 The sending SMTP server requires a username or password before forwarding the message. Check the SMTP server setup to ensure that the proper username and password have been inserted.
534 Message too large 5.3.4 The message exceeds the limits set by the sender’s or receiver’s systems. Reduce the email size or use a document sharing site (instead of attaching large files).
540 Issue with router or network 5.4.0 Interruption due to an undefined network issue.
541 Access denied 5.4.1 Commonly used reply by anti-spam filters Check the message’s spam score or sender’s reputation with spam filters; you can also ask the intended receiver to “whitelist” your sending email address.
542 Bad network connection 5.4.2 The sender or sender’s server is not connected to the internet. Check connections to the internet.
543 Network issues 5.4.3 Usually involves problems with a router. Check local routers to ensure they are operating properly.
544 Network issues 5.4.4 Usually involves problems with a router. Check local routers to ensure they are operating properly.
546 Network issues 5.4.6 Usually involves problems with a router, often involving a routing loop. Check local routers to ensure they are operating properly.
547 Network issues 5.4.7 The allotted time for delivery of command has expired.
550 Mailbox is unavailable 5.5.0 The target email address does not exist or is not available to the sent command. Check the email address; but also check for spam filter issues.
551 User not local 5.5.1 The target email is not on the receiving server, which often provides a forwarding address (but doesn’t relay the message automatically). Check the email address to ensure that the right address is being used.
553 Bad target email address 5.5.3 The target email address is invalid. Check the spelling or formatting (such as missing @) in the email address.
554 Transaction failed. 5.5.4 The receiving email server has rejected the email or sender, usually due to spam issues. Check the message’s spam score or sender’s reputation with spam filters; you can also ask the intended receiver to “whitelist” your sending email address.
571 Authentication required or refused 5.7.1 The receiving email server has rejected the email address, domain name or ISP. Check the message’s spam score or sender’s reputation with spam filters; you can also ask the intended receiver to “whitelist” your sending email address.

 

 

 

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