What is Bounce Rate?

Definition

Bounce rate is the share of sent messages that bounce, meaning they are rejected by the recipient's mail server and never reach an inbox. When an email cannot be delivered, the receiving server returns a notification, often called a bounce message or a non-delivery report, and your sending platform records that message as a bounce. The bounce rate rolls all of those failures into a single percentage for a campaign or a time period.

The calculation is straightforward. Bounce rate equals the number of bounced emails divided by the total number of emails sent, multiplied by 100. If you send 5,000 messages and 100 of them bounce, your bounce rate is 2 percent. Bounces are usually split into two categories that behave very differently. A hard bounce is a permanent failure, such as an address that does not exist, and it should be removed from the list immediately. A soft bounce is a temporary failure, such as a full mailbox or a busy server, and it may succeed on a later attempt. Tracking both, but watching the hard bounce figure most closely, gives the truest picture of list health.

How It Works

Bounces are generated by the Simple Mail Transfer Protocol, the system mail servers use to exchange messages. When your sending server hands a message to the recipient's server, the recipient's server replies with a status code. A successful acceptance uses a code in the 200 range. A failure uses a code that signals the problem, and the first digit of that code tells you the category.

Codes that begin with 5, such as 550 or 553, indicate a permanent failure and produce a hard bounce. The mailbox does not exist, the domain is not configured to receive mail, or the server has flatly refused the message. Codes that begin with 4, such as 421 or 452, indicate a temporary failure and produce a soft bounce. The mailbox might be over quota, the server might be busy, or the message might have been deferred by greylisting. Many systems also use enhanced status codes with a three-part structure, such as 5.1.1, where the additional digits pinpoint the exact reason for the failure.

Your email platform reads these responses, classifies each one, and aggregates them into the bounce rate you see in reporting. Well-run sending systems retry soft bounces a few times over a couple of days before giving up, and they suppress hard-bounced addresses automatically so the same dead address is not mailed again. The trouble is that by the time a bounce appears in a report, the damage to your sending reputation has already begun, since the receiving provider saw the failed delivery attempt in real time.

Why It Matters for Email Deliverability

Bounce rate is one of the most important signals mailbox providers use to judge a sender. A list full of valid addresses produces almost no bounces, which is what a careful, permission-based sender looks like. A list with many dead addresses produces a stream of hard bounces, which is a hallmark of spammers and senders working from old or purchased data. Providers respond to a high bounce rate by lowering the sender's reputation, which pushes more of that sender's legitimate mail into the spam folder.

The acceptable range is narrow and has tightened. A total bounce rate under 3 percent is generally healthy. A rate between 3 and 5 percent means the list needs cleanup, and a rate above 5 percent can do real, lasting damage to a sending reputation. The hard bounce figure should stay as low as possible for a well-managed campaign. These thresholds are not abstract best practice: mailbox providers treat elevated bounces as a sign that a sender is working from stale or poorly sourced data.

How VeriMails Handles It

The most reliable way to keep a bounce rate low is to never send to a bad address in the first place, and that is exactly what VeriMails is built to do. Before you mail a list, VeriMails checks each address through syntax validation, an MX and DNS lookup, a live SMTP handshake, catch-all detection, disposable address detection, and role-based detection. Addresses that would hard bounce, such as mailboxes that no longer exist or domains with no mail server, are identified so you can remove them ahead of the send.

VeriMails performs catch-all detection rather than scoring, so accept-all domains, which carry elevated bounce risk because the existence of the specific mailbox cannot be confirmed, are flagged plainly for you to handle. You can verify addresses one at a time through the REST API as contacts enter your system, or clean a whole list with a bulk CSV upload before a campaign. Results return as clear deliverability categories. Verification starts at $0.0019 per email, with 10,000 credits for $19 and subscriptions from $15 per month, and every account begins with 100 free credits, no credit card required, that never expire, so you can measure the bounce reduction on your own list before paying anything.

Frequently Asked Questions