Email Definitions: Bounces

A key concept in email is that of a bounce. A bounce is usually an indication that the message you sent was rejected by the intended recipient’s email infrastructure and may not have reached the recipient.

Hard and Soft Bounces

A hard bounce indicates a persistent delivery failure (e.g., mailbox does not exist). In other words, your recipient did not receive your email message, and your server will not try to resend it.

Unlike a hard bounce, a soft bounce is not a permanent failure or rejection (e.g., mailbox full). Many email systems (including Amazon SES) will automatically try to resend a message that has generated a soft bounce over a period of time until the message either delivers or the system will no longer retry the delivery and in turn generates a hard bounce.

Synchronous and Asynchronous Bounces

You may receive hard and soft bounces either asynchronously or synchronously. A synchronous bounce is communicated while the sender’s and the receiver’s email servers are actively transmitting and receiving the email message. You will receive a synchronous bounce immediately during your attempt to send.

Conversely, an asynchronous bounce is sent after the message is initially successfully accepted for delivery by the receiver. You may receive this type of bounce hours after you initially send your message.

Bounce Rate

Bounce rate usually refers to the number of hard bounces per number of emails sent, and is expressed as a percentage:

For example, if we sent 10,000 emails and 222 of them bounced, our bounce rate would be 222 ÷ 10,000, or 2.22%.

A usual best practice is to keep your bounce rate below 5% – any higher may indicate to Internet Service Providers (ISPs) that you are sending to an old or bought email list, which they frown upon.This may change with industry trends and isn’t universal across all ISPs, but it’s a good rule of thumb.

For more information on bounce codes and what they mean, please refer to RFC 3463.