Bounce and complaint rates: why 4% and 0.08% matter, and how automatic pausing protects you
Two numbers decide much of what happens to your email: your bounce rate and your complaint rate. Keep them low and mailbox providers learn to trust your domain. Let them rise and your mail starts going to spam, then gets throttled, and eventually your sending provider steps in.
Sendonis, which is launching soon, will watch both numbers for every account. If your bounce rate goes above 4% or your complaint rate goes above 0.08%, sending for your account will pause automatically. This post explains what the numbers measure, why we chose those limits, and what happens if you hit one.
What a bounce is
A bounce is a message the receiving side couldn't deliver. There are two kinds:
- Hard (permanent) bounces. The address doesn't exist, the domain doesn't exist, or the receiving server has permanently refused the message. Retrying won't help.
- Soft (transient) bounces. A temporary problem, such as a full mailbox, a server that's briefly unavailable, or a message that's too large. These may succeed later.
The bounce rate that matters for reputation is mostly about hard bounces: the share of your messages sent to addresses that don't work. A high hard-bounce rate tells mailbox providers that you aren't maintaining your list. That pattern is typical of old, purchased or scraped lists, which is why they treat it as a warning sign.
When an address hard-bounces through Sendonis, we'll add it to your account's suppression list automatically, and future sends to it will be blocked.
What a complaint is
A complaint is recorded when a recipient clicks "Report spam" (or a similar button) on your message. Many mailbox providers run feedback loops that report these clicks back to the sender. Our sending infrastructure receives those reports, and Sendonis will match them to your account, suppress the address and send you a webhook event.
Complaints are a direct signal from people that they didn't want your email. Mailbox providers weigh them heavily. Two details make the complaint rate stricter than it looks:
- It's measured against all mail you send, not just the messages people opened.
- Not every provider reports individual complaints, so the rate you can measure may understate the real one. If your measured rate is creeping up, the true picture is probably worse.
Why these thresholds exist at all
Large email infrastructure providers monitor bounce and complaint rates for every sending account. The infrastructure Sendonis sends through, for example, may place an account under review at a bounce rate of around 5% or a complaint rate of around 0.1%, and may pause sending at around 10% bounces or 0.5% complaints. Large mailbox providers set their own expectations too. Google, for example, tells bulk senders to keep the spam rate reported in Postmaster Tools below 0.3%, and ideally well below 0.1%.
Sendonis is a multi-tenant platform. All Sendonis customers send through one dedicated sending account. If one tenant's bad list pushed the account's overall rates past those review levels, every other customer could be affected. That's not acceptable, so we'll enforce limits per tenant that sit below the infrastructure's review levels:
| Metric | Infrastructure review level (approx.) | Sendonis automatic pause |
|---|---|---|
| Bounce rate | 5% | above 4% |
| Complaint rate | 0.1% | above 0.08% |
The margin between our limits and the infrastructure's limits gives us time to act on one account before the platform as a whole is at risk.
What the numbers mean in practice
It helps to turn percentages into counts:
- 4% bounces = 4 hard bounces for every 100 messages, or 400 per 10,000.
- 0.08% complaints = 8 spam reports for every 10,000 messages.
For a healthy transactional sender, both limits are generous. Password resets and receipts go to addresses people just typed in and want to hear from, so bounces and complaints should be rare. Senders usually hit these limits for one of a few reasons:
- importing an old list that hasn't been mailed in years;
- sending to a purchased, rented or scraped list, which our Acceptable Use Policy prohibits;
- a sign-up form without validation, which lets typos and fake addresses in;
- sending marketing content to people who only signed up for account emails;
- a bug that sends the same message repeatedly, or to the wrong audience.
Rates will be calculated per account over a defined measurement window, with a minimum number of messages, so a handful of bounces on a very small send doesn't trigger a pause. Before you reach the limits, we may also contact you with an early warning.
What happens when sending pauses
From launch, if your account crosses either limit:
- Sending stops for your account. Other Sendonis customers aren't affected.
- You're notified with the metric that triggered the pause.
- We review the account together. Depending on the cause, we may ask what changed, request evidence of consent for the affected list, or ask you to clean the list or send at a reduced volume at first.
- Sending resumes once the cause is fixed.
A pause isn't a penalty. It's a circuit breaker. Stopping after a few thousand bad messages is much better for your domain than letting a campaign run to the end and finding out from your spam-folder rate a week later.
How to stay well below the limits
Most of this is simple list hygiene:
- Only send to people who asked. Marketing email needs opt-in consent. Transactional email should go only to people who have a relationship with you.
- Validate at the point of entry. Check address syntax, consider confirming new sign-ups with a link, and don't let bots fill your forms.
- Never re-import suppressed addresses. If an address bounced or complained, it stays suppressed unless the person gives fresh, documented consent.
- Retire old lists. If you haven't emailed a list in a long time, ask for permission again before you use it.
- Make unsubscribing easy. A clear unsubscribe link costs you one recipient. A spam complaint costs you reputation.
- Separate streams. Don't mix promotional content into transactional messages.
- Watch your webhooks. Bounce and complaint events arrive as they happen. A sudden spike after a deploy or an import is your earliest warning.
Shared reputation, shared protection
Every sender on a shared platform benefits when the others behave well. The 4% and 0.08% limits, automatic suppression and our strict AUP all serve the same purpose: keeping Sendonis a platform whose mail mailbox providers trust. That trust is what gets your password resets and receipts into the inbox.
Read more in our Acceptable Use Policy. Want to hear when Sendonis launches? Join early access.