Email bounce rate measures the percentage of sent emails that could not be delivered. It is one of the core health metrics for any email program and directly affects sender reputation and deliverability. Keep your overall bounce rate below 2% and your hard bounce rate below 0.5%.
Hard bounces occur when an email address is permanently undeliverable: the address does not exist, the domain is invalid, or the server has permanently rejected the message. Hard bounces should immediately suppress the address from future sends. Continuing to send to hard-bounced addresses signals to mailbox providers that you are not maintaining your list.
Soft bounces are temporary failures: the recipient's mailbox is full, the server is temporarily down, or the message is too large. Most MAPs retry soft bounces a set number of times. After repeated soft bounces (typically 3 to 5 attempts), the address should be reclassified as a hard bounce and suppressed.
High bounce rates indicate list quality problems. Common causes include old data (people change jobs, companies fold), purchased or rented lists (often contain invalid addresses), lack of double opt-in (typos and fake addresses get through), and insufficient list hygiene (no regular cleaning or validation).
Email verification tools like ZeroBounce, NeverBounce, and BriteVerify can validate email addresses before you send to them. Running verification on your database quarterly, and on every imported list before sending, significantly reduces bounce rates. The cost per verification is typically $0.003 to $0.01, which is trivial compared to the deliverability damage from high bounce rates.