Bounce Rate
The percentage of sent emails that were not delivered for any reason.
Bounce rate is the percentage of sent emails returned as undeliverable. A hard bounce is a permanent failure, such as an address or domain that does not exist; SMTP reports these with 5.x.x status codes (RFC 3463). A soft bounce is temporary, such as a full mailbox or a server that is down, and uses 4.x.x codes. Email platforms suppress hard-bounced addresses automatically and retry soft bounces for a period before giving up.
How to measure bounce rate
Bounce rate = bounces ÷ emails sent × 100. Look at hard and soft bounces separately. Hard bounces point to list quality: typos, abandoned addresses or contacts imported from an old source. A soft bounce on a single campaign is usually noise, but repeated soft bounces at one mailbox provider can mean it is throttling or blocking you; the bounce message text usually says which.
Each email platform sets its own acceptable bounce level in its terms, and a high rate on an imported list can get a campaign paused or an account reviewed.
Worked example
Send to 5,000 contacts, get 60 hard bounces and 40 soft bounces, and the bounce rate is 100 ÷ 5,000 = 2%, with a hard bounce rate of 1.2%. The 60 hard-bounced addresses should be suppressed after this send. The 40 soft bounces are retried automatically, and many platforms convert an address to a hard bounce after it soft-bounces across several campaigns in a row.
Common bounce rate mistakes
Re-importing contacts who previously hard-bounced or unsubscribed. Sending to a list that has sat unused for a year without verifying it first. Treating block bounces, rejections whose message mentions policy, spam or reputation, as a list problem when they are a reputation problem.