Understanding Bounces and Complaints

This guide explains bounce and spam complaint metrics in Sender — what they measure, how they are calculated, and how to interpret them to protect your sender reputation.

What These Metrics Measure

Bounce metrics track emails that were rejected by the recipient's mail server and never reached the inbox. Sender separates bounces into two types: hard bounces (permanent delivery failures) and soft bounces (temporary delivery failures). Spam complaint metrics track instances where recipients mark your email as spam or junk through their email client. Together, these metrics are critical indicators of list health, sending reputation, and long-term deliverability.

Metric Definitions

Hard bounce rate — The percentage of sent emails that permanently failed to deliver. Calculated as the number of hard bounces divided by total emails sent, expressed as a percentage. Displayed on the Dashboard within the Traffic and reach report and on the Campaign overview under Statistics as hard bounced.

Soft bounce rate — The percentage of sent emails that temporarily failed to deliver. Calculated as the number of soft bounces divided by total emails sent, expressed as a percentage. Displayed on the Dashboard within the Traffic and reach report and on the Campaign overview under Statistics as soft bounced.

Bounce rate — The combined percentage of all bounced emails (hard and soft) relative to total emails sent. Displayed on the Dashboard within the Traffic and reach report as a single aggregate percentage.

Total bounces — The raw count of all bounced emails (hard and soft combined) within the selected date range. Displayed on the Dashboard within the Traffic and reach report as an absolute number.

Total spams — The raw count of spam complaints received within the selected date range. Displayed on the Dashboard within the Traffic and reach report as an absolute number.

Average spam rate — The percentage of sent emails that generated spam complaints, averaged across campaigns in the selected date range. Displayed on the Dashboard within the Traffic and reach report as a percentage.

Spam reports — The per-campaign count and percentage of recipients who reported the email as spam. Displayed on the Campaign overview under Statistics and accessible as a detailed subscriber list under Subscriber actions → Spam reports.

How to Interpret the Data

Hard bounce rate below 0.5% — Indicates a clean, well-maintained list with valid email addresses. No immediate action is needed, but continue practicing routine list hygiene.

Hard bounce rate between 0.5% and 2% — Suggests that a portion of your list contains invalid or outdated addresses. Review recently added subscribers and check for data-entry errors, expired domains, or inactive accounts.

Hard bounce rate above 2% — Signals a serious list quality problem that can damage your sender reputation. Investigate the source of these addresses and remove or suppress hard-bounced contacts immediately.

Soft bounce rate below 2% — Within normal range. Soft bounces are typically caused by temporary conditions such as full inboxes or server downtime, and often resolve on their own.

Soft bounce rate between 2% and 5% — May indicate recurring delivery issues with certain mailbox providers or that some recipients' inboxes are consistently full. Monitor whether the same addresses soft-bounce repeatedly across campaigns.

Soft bounce rate above 5% — Warrants investigation. Persistent soft bounces to the same addresses may eventually be treated as hard bounces by mailbox providers, and a high soft bounce rate can signal infrastructure or content-related filtering.

Spam complaint rate below 0.1% — Generally considered acceptable by most mailbox providers. This is the threshold many providers use to evaluate sender reputation, so staying well below it is important.

Spam complaint rate between 0.1% and 0.3% — A warning range. Some mailbox providers may begin throttling or filtering your emails. Review your content, sending frequency, and whether recipients clearly opted in.

Spam complaint rate above 0.3% — Critical. Sustained rates at this level can trigger inbox placement penalties, domain reputation damage, or sending restrictions. Immediate action is needed to identify the cause.

Note that all of these thresholds are general references. Acceptable ranges vary by industry, audience size, and mailbox provider tolerance levels.

How Metrics Relate to Each Other

Hard bounces and delivery rate — Every hard bounce is an email that was never delivered. A rising hard bounce rate directly reduces your delivery rate and indicates addresses that should be removed from your list to avoid compounding reputation damage.

Soft bounces and hard bounces — Repeated soft bounces to the same address across multiple campaigns may eventually be reclassified as hard bounces by the receiving mail server. Monitoring both metrics together helps you identify contacts that are persistently unreachable.

Bounce rate and spam complaint rate — Both metrics feed into your sender reputation as assessed by mailbox providers. High bounces suggest poor list quality, while high complaints suggest recipients did not want the email. When both are elevated simultaneously, the reputation impact is compounded.

Spam complaints and open rate — A spike in spam complaints often correlates with changes in content or sending frequency that recipients did not expect. If your complaint rate rises while open rates decline, recipients may be disengaging before ultimately marking messages as spam.

Bounce rate and total emails delivered — The total emails delivered count on the Campaign overview equals total emails sent minus all bounces (hard and soft). A high bounce rate directly reduces the pool of delivered emails, which in turn limits the audience available for opens and clicks.

Tracking Limitations

Feedback loop coverage — Spam complaints are reported through feedback loops (FBLs) provided by mailbox providers. Not all providers operate FBLs, and some (notably Gmail) do not return individual complaint data to senders. The Spam reports count in Sender reflects only complaints received through available feedback loops and may undercount actual complaints.

Bounce classification accuracy — Bounce codes returned by mail servers are not always standardized. Some servers return vague or non-standard error codes, which may occasionally lead to a soft bounce being classified differently than expected, or vice versa. Sender categorizes bounces based on the response codes received.

Delayed bounce reporting — Some bounces, particularly soft bounces, may not be reported immediately by the receiving mail server. This means bounce counts for a campaign may continue to update for a period after sending, and initial statistics may not reflect the final bounce totals.

Complaint attribution timing — A recipient may report an email as spam days or weeks after receiving it. The complaint is attributed to the original campaign, but the Date / time recorded in the Subscriber actions → Spam reports view reflects when the complaint was processed, not when the email was originally opened or read.

Common Issues

Hard bounce rate spikes after importing a new list → The imported list likely contains invalid, outdated, or mistyped email addresses. Clean the list by removing hard-bounced addresses and consider using an email verification service before importing future lists.

Same addresses soft-bounce repeatedly across campaigns → The recipient's mailbox may be permanently full or abandoned, or the mail server may be consistently rejecting your messages. If an address soft-bounces across three or more consecutive campaigns, consider suppressing it to prevent further delivery attempts.

Spam complaint rate suddenly increases → This typically follows a change in sending frequency, content style, or audience targeting. Review what changed in recent campaigns — a new segment, higher send volume, or different content format may be driving complaints from recipients who did not expect the message.

Bounce rate is high but all addresses were confirmed subscribers → Email addresses can become invalid over time due to account closures, domain expirations, or corporate email migrations. Even a confirmed list degrades if not regularly maintained. Implement routine list cleaning to remove addresses that have hard-bounced.

Spam reports show zero but you suspect complaints exist → Not all mailbox providers return complaint data through feedback loops. Gmail, for example, does not provide individual complaint reports. A zero count in Spam reports does not guarantee zero complaints — it means no complaints were returned through available feedback loops.

FAQs

What is the difference between a hard bounce and a soft bounce?

A hard bounce is a permanent delivery failure — the email address does not exist, the domain is invalid, or the server has permanently rejected the message. A soft bounce is a temporary delivery failure caused by conditions such as a full mailbox, a temporarily unavailable server, or a message that exceeds size limits. Sender tracks both types separately and displays them as hard bounced and soft bounced on the Campaign overview, and as Hard bounce rate and Soft bounce rate on the Dashboard.

What bounce rate should I be concerned about?

A combined Bounce rate above 2% per campaign warrants investigation. Hard bounces above 0.5% suggest invalid addresses in your list. Consistent high bounce rates can damage your sender reputation and reduce inbox placement over time. Benchmarks vary by industry and list age.

What happens to contacts that hard-bounce?

When an email address generates a hard bounce, Sender records it in the Subscriber actions → Hard bounces list for that campaign. Hard-bounced addresses are typically flagged to prevent further sending, protecting your sender reputation from repeated failed delivery attempts.

Does a spam complaint automatically unsubscribe the recipient?

Yes. When a spam complaint is received through a feedback loop, Sender processes the complaint and suppresses future sends to that recipient. The complaint is recorded in the Subscriber actions → Spam reports view for the associated campaign.

Why is my spam complaint rate showing 0% when I know recipients are unhappy?

The Average spam rate and Spam reports metrics only reflect complaints returned through mailbox provider feedback loops. Providers like Gmail handle complaints internally and do not report them individually back to senders. A 0% spam rate does not confirm zero complaints — it means no complaint data was received through available channels.

Can I see which specific subscribers bounced or complained?

Yes. On the Campaign overview page, expand Subscriber actions in the left sidebar and select Hard bounces, Soft bounces, or Spam reports to view a list of affected email addresses with timestamps. You can also search by email address and export the data.

How does Sender calculate the Bounce rate on the Dashboard?

The Bounce rate on the Dashboard is calculated as total bounces (hard and soft combined) divided by total emails sent across all campaigns within the selected date range, expressed as a percentage. The Hard bounce rate and Soft bounce rate break this down by bounce type.