What is a hard bounce/soft bounce?

When an ISP does not accept a message it will result in a bounce. Bounces contain numbers and text explaining why the message was not delivered. Bounces are categorized as a Hard Bounce or a Soft Bounce.

A Hard Bounce occurs when the message did not reach the recipient because the address does not exist due to an invalid address, an unknown user or a bad domain. Hard bounces are permanent failures and do not get retried as sending mail to a hard bounced address will not result in a successful delivery. The number of Hard Bounces on a send speaks to the hygiene of the list. High Hard Bounces will impact a sender's reputation and ability to inbox. Anything above 1% Hard Bounces would be considered high and should be investigated. The average Hard Bounce rate seen is 0.05%.

A Soft Bounce occurs when the message did not reach the recipient due to a temporary situation such as network issues, spam filter rejections, mailbox full or other temporary scenario. Soft bounces are temporary and will be tried with the next send attempt. Soft bounced addresses typically result in successful delivery of subsequent messages. If your soft bounce rate has increased significantly on one day, particularly at one ISP specifically, your mail may be getting blocked due to sender reputation issues and action should be taken to rebuild your reputation.

How many Soft Bounces until a user becomes a Hard Bounce?

A Soft Bounce will never turn into a Hard Bounce. Sailthru will continue to try to send messages to recipients that have soft bounced for all future message sends.

A user who Soft Bounces is still a valid user. Given valid users are of value to a customer, a Soft Bounce will not turn into a Hard Bounce after x consecutive Soft Bounces. If a user has an engagement based suppression list then any user who continually Soft Bounces will eventually fall into the suppression list and stop being messaged. In most cases a user who Soft Bounces consecutively is because 1) the mailbox is overquota/full (in this case the ISP will eventually Hard Bounce out the user after giving them time to clear out the inbox) 2) It's a user at a small tier 3 ISP that is blocking the mail. Customers can pull lists of users and the associated bounce codes which tell the reason for the Soft Bounce.