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, Outlook.com, Postini and Yahoo!‘s postmaster sites for additional bounce code definitions.
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