Email: Soft Bounce and Hard Bounce Code Lookup and Definitions

Email Soft Bounce and Email Hard Bounce Code Lookups 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

Code Type Description
421 Soft Service not available
450 Soft Mailbox unavailable
451 Soft Error in processing
452 Soft Insufficient 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

Code Type Description
500 Hard Address does not exist
510 Hard Other address status
511 Hard Bad destination mailbox address
512 Hard Bad destination system address
513 Hard Bad destination mailbox address syntax
514 Hard Destination mailbox address ambiguous
515 Hard Destination mailbox address valid
516 Hard Mailbox has moved
517 Hard Bad sender’s mailbox address syntax
518 Hard Bad sender’s system address
520 Soft Other or undefined mailbox status
521 Soft Mailbox disabled, not accepting messages
522 Soft Mailbox full
523 Hard Message length exceeds administrative limit
524 Hard Mailing list expansion problem
530 Hard Other or undefined mail system status
531 Soft Mail system full
532 Hard System not accepting network messages
533 Hard System not capable of selected features
534 Hard Message too big for system
540 Hard Other or undefined network or routing status
541 Hard No answer from host
542 Hard Bad connection
543 Hard Routing server failure
544 Hard Unable to route
545 Soft Network congestion
546 Hard Routing loop detected
547 Hard Delivery time expired
550 Hard Other or undefined protocol status
551 Hard Invalid command
552 Hard Syntax error
553 Soft Too many recipients
554 Hard Invalid command arguments
555 Hard Wrong protocol version
560 Hard Other or undefined media error
561 Hard Media not supported
562 Hard Conversion required and prohibited
563 Hard Conversion required but not supported
564 Hard Conversion with loss performed
565 Hard Conversion failed
570 Hard Other or undefined security status
571 Hard Delivery not authorized, message refused
572 Hard Mailing list expansion prohibited
573 Hard Security conversion required but not possible
574 Hard Security features not supported
575 Hard Cryptographic failure
576 Hard Cryptographic algorithm not supported
577 Hard Message integrity failure

5XX Soft Bounce and Hard Bounce Code Lookup and Definitions

Code Type Description
911 Hard Hard 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,, Postini and Yahoo!‘s postmaster sites for additional bounce code definitions.