Email: Soft Bounce and Hard Bounce Code Lookup and Definitions

An email bounce is when an email isn’t accepted by a business or Internet Service Provider’s mail server for a specific email address and a code is returned that the message has been rejected. The bounces are defined as either soft or hard. Soft bounces are typically temporary and are basically a code to tell the sender that they may wish to keep trying. Hard bounces are typically permanent and are coded to tell the sender to not attempt to try to send the message again to the recipient.

Definition of a Soft Bounce

A soft bounce is a temporary indicator of an issue with the recipient’s email address. It means the email address was valid, but the server rejected it. Typical reasons for a soft bounce are a full mailbox, a server outage, or the message was too large. Most email service providers will re-attempt sending the message to multiple times over a period of several days before giving up. They may or may not block the email address from being sent again.

Definition of a Hard Bounce

A hard bounce is a permanent indicator of an issue with the recipient’s email address. It means that, most likely, the email address was not valid and the server permanently rejected it. It could have been a malformed email address or an email address that didn’t or no longer exists on the recipient’s mail server. Email service providers will typically block these email addresses from being sent to again. Sending repeatedly to a hard bounced email address can get your email service provider blacklisted.

4XX Soft Bounce and Hard Bounce Code Lookup and Definitions

CodeTypeDescription
421SoftService not available
450SoftMailbox unavailable
451SoftError in processing
452SoftInsufficient system storage

As one of our commenters noted below, the actual RFC associated with email delivery and return codes specifies that codes in 5.XXX.XXX format are Permanent Failures, therefore a designation of hard codes may be appropriate. The issue isn’t the returned code, it’s how you should treat the source email address. In the event of the codes indicated below, we’re indicating some codes as Soft.

Why? Because you may reattempt or send a new email to those recipients in the future and they would work absolutely fine. You may wish to add logic in your delivery to reattempt multiple times or across multiple campaigns. If the code persists, you may then update the email address as undeliverable.

5XX Soft Bounce and Hard Bounce Code Lookup and Definitions

CodeTypeDescription
500HardAddress does not exist
510HardOther address status
511HardBad destination mailbox address
512HardBad destination system address
513HardBad destination mailbox address syntax
514HardDestination mailbox address ambiguous
515HardDestination mailbox address valid
516HardMailbox has moved
517HardBad sender’s mailbox address syntax
518HardBad sender’s system address
520SoftOther or undefined mailbox status
521SoftMailbox disabled, not accepting messages
522SoftMailbox full
523HardMessage length exceeds administrative limit
524HardMailing list expansion problem
530HardOther or undefined mail system status
531SoftMail system full
532HardSystem not accepting network messages
533HardSystem not capable of selected features
534HardMessage too big for system
540HardOther or undefined network or routing status
541HardNo answer from host
542HardBad connection
543HardRouting server failure
544HardUnable to route
545SoftNetwork congestion
546HardRouting loop detected
547HardDelivery time expired
550HardOther or undefined protocol status
551HardInvalid command
552HardSyntax error
553SoftToo many recipients
554HardInvalid command arguments
555HardWrong protocol version
560HardOther or undefined media error
561HardMedia not supported
562HardConversion required and prohibited
563HardConversion required but not supported
564HardConversion with loss performed
565HardConversion failed
570HardOther or undefined security status
571HardDelivery not authorized, message refused
572HardMailing list expansion prohibited
573HardSecurity conversion required but not possible
574HardSecurity features not supported
575HardCryptographic failure
576HardCryptographic algorithm not supported
577HardMessage integrity failure

5XX Soft Bounce and Hard Bounce Code Lookup and Definitions

CodeTypeDescription
911HardHard bounce with no bounce code found It could be an invalid email or rejected email from your mail server (such as from a sending limit)

Some ISPs also have additional clarification in their bounce codes. See AOL, Comcast, Cox, Outlook.com, Postini and Yahoo!‘s postmaster sites for additional bounce code definitions.

3 Comments

  1. 1

    Hi, I’m a bit confused on how the email statuses are calssified based on the codes into either soft or hard bounces. Because here, in RFC 3463 (https://tools.ietf.org/html/rfc3463) it says that codes in 4.XXX.XXX format are Persistent Transient Failures which means they fall into the soft bounce category and codes in 5.XXX.XXX format are Permanent Failures, which means they fall under hard bounces.
    Can you clarify why there are some status codes which begin from 5 are classified as soft bounces in this article?

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