What Are Email Bounce Codes?

Bounce codes are messages from a mail server to indicate why an email you sent could not be delivered.

Bounce codes indicate whether the failure was a soft bounce (i.e., a temporary issue) or a hard bounce (i.e., a permanent issue).

The codes also shed light on the specific problem, which can range from the recipient having a full mailbox to an email account no longer existing.

Soft Bounce vs. Hard Bounce

Soft bounces are emails that bounce for a temporary reason. For example, the recipient's mailbox may be full, or there may be an error on their mail server.

Hard bounces are messages that bounce for a permanent reason. For example, the recipient's mailbox no longer exists.

The general rule is to try resending to addresses that temporarily bounce and stop trying to send to addresses that permanently bounce, both in this campaign and in the future.

Enhanced SMTP Bounce Codes vs. Traditional SMTP Bounce Codes

Traditional SMTP bounce codes are three-digit numbers (e.g., 554) that indicate whether an email was delivered successfully or not. If the delivery was unsuccessful, the codes indicate the reason.

For example, status code 550 means that a user's mailbox was invalid.

Codes that start with 4 (e.g., 421) usually indicate a soft bounce; codes that start with 5 (e.g., 550) indicate a hard bounce.

Enhanced bounce codes include three distinct digits separated by periods, allowing bounce notifications to offer more specific details about why the message delivery failed.

Example Enhanced Bounce Codes:

- 550 5.7.1 Email rejected due to security policies.
- 550 5.7.26 Unauthenticated email from the domain is not accepted due to the domain’s DMARC policy.
- 551 5.7.1 Message rejected due to its classification as virus, spam, or high bounce rate.
- 552 5.2.2 The message exceeds the current system limits.
- 553 5.1.2 We couldn’t find the recipient domain.
- 554 5.6.0 The email message is malformed.
- 554 5.7.0 Too many unauthenticated commands.
- 555 5.5.2 Syntax error.
- 561 5.6.1 Unsupported media type.
- 571 5.7.1 Delivery not authorized, message rejected.
- 573 5.7.3 Security conversion required but not possible.
- 575 5.7.5 Cryptographic failure.
- 576 5.7.6 Unsupported cryptographic algorithm.

Updated on: 09/03/2024