What is a hard bounce?

Every hard bounce tells a mailbox provider that you sent to an address you had not checked. A handful is normal on any list. A steady stream says the list is old, bought or badly collected, and that is what damages a sender’s reputation.

What causes a hard bounce

• A typo at sign-up, such as gmial.com instead of gmail.com.
• A mailbox that was closed or never existed.
• A domain that does not exist or has no mail server.
• A list that was scraped, bought or left untouched for years.

The receiving server answers with a permanent error, in the 5xx range. The SMTP error codes entry shows which codes mean what.

Hard bounces in Amazon SES

SES labels these bounces as permanent. The bounce notification carries a subtype that tells you more:

Subtype	Meaning
General	The recipient’s email provider sent a hard bounce message.
NoEmail	SES could not retrieve the recipient address from the bounce message.
Suppressed	The address is on the SES suppression list because it has a recent history of producing hard bounces.
OnAccountSuppressionList	SES did not send because the address is on your own account-level suppression list. This does not count toward your bounce rate.

Permanent bounces to domains you have not verified count toward your bounce rate. On accounts created after 25 November 2019, SES also adds the address to your account-level suppression list by default, so it will not send to that recipient again. Older accounts have to switch this on.

What to do about them

Stop sending to the address. Then look for the cause: a sign-up form that does not confirm addresses, a long-dormant list, or an import from a source you cannot vouch for. Confirming addresses with a double opt-in email catches typos before they ever reach your sending rate.