A bounce happens when a message cannot be delivered and the receiving server sends an error response back to the sender. Your ESP records this as a bounce event. The nature of the error — permanent vs. temporary — determines whether you should immediately suppress the address or retry.
Bounce rate is one of the most closely-watched deliverability signals. Mailbox providers and ESPs use it as a proxy for list quality. A high hard bounce rate signals that you are sending to addresses that don't exist — which is a strong indicator of poor list hygiene, list purchases, or stale data.
Hard bounce vs. soft bounce
A hard bounce is a permanent delivery failure with a 5xx SMTP status code. Common reasons: the email address does not exist (550 5.1.1 User unknown), the domain does not exist (DNS lookup failure), or the receiving server has permanently blocked your IP or domain. Hard-bounced addresses should be immediately and permanently suppressed — continuing to send to them harms your reputation.
A soft bounce is a temporary delivery failure with a 4xx SMTP status code. Common reasons: the recipient's mailbox is full (452), the receiving server is temporarily unavailable, or the message was too large. ESPs typically retry soft bounces for 24–72 hours before converting them to a bounce event. A soft bounce on the same address across multiple campaigns is often a signal the address is abandoning — it is worth suppressing after 2–3 consecutive soft bounces.
Bounce rate thresholds
The industry threshold for hard bounce rate is 2%. Gmail and Outlook both treat sustained hard bounce rates above 2% as a signal of poor list quality and will apply increased filtering. SendGrid, Mailchimp, and most ESPs will temporarily suspend sending if hard bounce rate crosses 5% in a single campaign.
Hard bounce rate is calculated as hard bounces divided by messages sent. A single campaign to a poor list can spike this metric — which is why pre-import verification matters. Even a 5% hard bounce rate on one import will pull your rolling rate above acceptable thresholds if the import volume is significant relative to your normal sending volume.
Frequently asked questions
01Should I remove bounced email addresses from my list?
Hard bounces: yes, immediately and permanently. Sending to a hard-bounced address again signals to mailbox providers that you are not maintaining list hygiene. Most ESPs automatically suppress hard bounces — verify that your ESP's suppression is active and that imported lists are checked against the suppression list before sending.
02What is the difference between a bounce and a rejection?
A bounce is a delivery failure where the receiving server accepted the SMTP connection but then returned an error (4xx or 5xx) during or after message transfer. A rejection is an earlier failure where the receiving server refused the connection or the RCPT TO command before the message was transmitted. ESPs often report both as 'bounces' even though technically they are distinct events.
03Can bounce rate affect deliverability to other recipients?
Yes. Your bounce rate is measured across your sending IP and domain, not per-recipient. A high bounce rate from sending to one poor list degrades your domain and IP reputation for all subsequent sends — including to your engaged, opt-in subscribers. This cross-contamination effect is why list hygiene should be applied before sending, not after.
Run a free deliverability audit on your sending domain.
MailerMonk checks DMARC alignment, SPF lookups, DKIM keys, MX records, and major blocklists in under a minute. No signup, no card.