A soft bounce means the door was shut for now, not for good. The mailbox might be over its quota, the receiving server might be overloaded, or the message might be too large. Retrying later often works.
Common causes
- The recipient’s mailbox is full.
- The receiving server is down or throttling you.
- The message is larger than the recipient allows.
- The content was rejected by a filter, which is the one cause worth taking seriously.
The server answers with a temporary error in the 4xx range. See SMTP error codes for the common ones.
Soft bounces in Amazon SES
SES reports these as transient bounces. The subtypes you will see are General, MailboxFull, MessageTooLarge, ContentRejected and AttachmentRejected.
Only hard bounces count toward the bounce rate that AWS reviews, so a soft bounce does not push you toward a review on its own. That does not make them noise. A rise in ContentRejected often means a mailbox provider has started to dislike what you send, and that shows up in your sender reputation well before it shows up in a rate.
When to act
Do nothing about a single soft bounce. Look closer when the same recipient soft bounces on several sends in a row, since a mailbox that stays full is effectively dead, or when one subtype climbs across many recipients at once.
Common questions
Do soft bounces count toward the SES bounce rate?
No. AWS says temporary failures such as a full mailbox, and bounces caused by blocked IP addresses, do not count toward your bounce rate.
Can soft bounces hurt my reputation?
A pattern of them can point at a content or reputation problem, even though they stay out of the rate. Isolated ones are usually harmless.