What is considered a soft bounce on Amazon SES, and how can I monitor soft bounces?

Resolution

A soft bounce or a transient bounce is a bounced email that's rejected by the recipient mail server because of a temporary failure. Temporary failures that can lead to a soft bounce include:

• The recipient mailbox is full.
• The recipient mailbox is temporarily unavailable.
• The server limits are exceeded.
• The server is overloaded.

It's best practice to retry soft bounces, which are caused by temporary issues.

Note: If you're using Simple Mail Transfer Protocol (SMTP) with Amazon SES, then the specific error codes related to soft bounces are 421, 450, 451, or 452. For the descriptions of these error codes, see Simple Mail Transfer Protocol (SMTP) Enhanced Status Codes Registry on the Internet Assigned Numbers Authority (IANA) website.

Amazon SES categorizes soft bounces or transient bounces as these subtypes:

• General: The recipient's email provider sent a general transient bounce. This typically happens when the recipient has set up an "out of the office" automatic reply. This type of soft bounce generally fixes itself. If the bounce was a result of an "out of the office" reply, then the soft bounce resolves itself after the automatic reply is removed by the recipient.
• MailboxFull: The recipient mailbox is full and can't accept any more emails. You might be able to send emails to the same recipient later when their mailbox is no longer full.
• MessageTooLarge: The recipient mail server can't accept the email because the file size of the message is too large. You can try to resolve this type of bounce by reducing your message size.
• ContentRejected: The recipient mail server doesn't allow the content of the email. You can try to resolve this type of bounce by modifying the content of your email.
• AttachmentRejected: The email contains an unacceptable attachment. Some email providers block emails with attachments of a certain file type or large attachments. After you remove or modify your email attachments, you can try to send the email to this recipient again.

For more detail on these soft bounce subtypes, see Bounce types.

To monitor soft bounces on Amazon SES, you can set up notifications using Amazon Simple Notification Service (Amazon SNS). The SNS notifications are triggered whenever any type of bounce is received.

To determine if the rejection is a soft bounce, review the content of the SNS notification. If bounceType is Transient, then it's a soft bounce. To determine the type of soft bounce, review the value for bounceSubType.