Bounce Rate

Bounce rate is the proportion of a send that came back undelivered. You take the number of bounced messages, divide by the number you sent, and multiply by 100. Most ESPs report it automatically per campaign, but the maths is simple enough to sanity-check by hand:

Not every bounce means the same thing, and the distinction drives what you do next:

• Hard bounce: a permanent failure, returned with a 5xx SMTP reply. The address does not exist, the domain is invalid, or the server has blocked you outright. Remove these addresses immediately; mailing them again is one of the strongest negative signals you can send.
• Soft bounce: a temporary failure, returned with a 4xx reply. The mailbox is full, the server is briefly down, or you are being throttled. Your mail server retries automatically; only suppress an address that soft-bounces repeatedly.

Because the two demand opposite responses, track them separately. A 1% rate that is almost entirely soft bounces from one overloaded provider is a very different problem from a 1% rate of hard bounces telling you a chunk of your list is dead.

The widely used rule of thumb is to keep your total bounce rate under 2%. Under 1% is excellent, 1% to 2% is acceptable, 2% to 5% deserves investigation, and anything above 5% is a serious list-quality problem that will start to throttle your delivery. Industry benchmark studies routinely put the cross-industry average right around the 2% line, so a rate well below it is a sign of healthy data.

A high rate is no longer just a vanity-metric problem. Gmail and Microsoft do not publish a bounce-rate cutoff, but a poorly maintained list drags down the reputation that decides placement, and the tightened 2025 sender rules key enforcement to authentication and a low spam-complaint rate. The single biggest lever is keeping a clean list through email verification before import and prompt suppression of every hard bounce. The full picture is in the guide to understanding bounce rates.

A high bounce rate tells mailbox providers that you are not maintaining your list, and that is a hallmark of spammers, who buy or scrape addresses and never clean them. Repeatedly hitting invalid mailboxes and dormant spam traps drags down both your domain and IP reputation, which then pushes good mail to the spam folder.

The damage compounds. Bounces reduce the denominator of every other engagement metric, and a reputation hit from one bad send can take weeks of careful sending to repair. This is why the cheapest insurance in email is never letting bad addresses onto the list in the first place, and removing them the instant they bounce.