Why is my warmup bounce rate high?

Short answer: in the large majority of cases, it isn't actually high — it just looks that way. A warmup bounce rate that seems alarming is almost always one of two harmless things: a small sample in your first days (where a single undelivered email is a big percentage), or a temporary hiccup on the receiving side that WarmySender doesn't even count against you. A genuinely high rate is rare, and when it happens there are a few simple things to check. This page walks through all of it, so you know exactly when to relax and when to take a quick look.

WarmySender is a 4-pillar outreach platform — Cold Emailing, Email Warmup, LinkedIn Outreach, and Multichannel sequences. This page covers an Email Warmup question: what your warmup bounce rate really means and when it's worth acting on.

What "bounce rate" means in warmup

Your warmup bounce rate is simply the share of your warmup emails that couldn't be delivered. If 100 warmup emails go out and 2 can't be delivered, that's a 2% bounce rate. Lower is better — a low bounce rate means your warmup mail is reaching real, working inboxes, which is exactly what builds and protects your sender reputation.

The key thing to hold onto is that bounce rate is a percentage, not a raw count. And percentages behave in a way that can be misleading when the number of emails is still small — which is the single most common reason the figure looks high early on.

Why it can look high at the start — the small-sample effect

In your first days of warmup, only a handful of emails have been sent. When the total is small, a single undelivered email turns into a surprisingly big percentage — even though, in real terms, it's just one email.

A worked example makes it obvious:

• 1 undelivered email out of 16 sent shows as 6.3% — which can look worrying at a glance. But that "6.3%" is really just one email that didn't land. Nothing about your mailbox is wrong.
• As your daily volume grows over the coming days, that same one-off barely moves the needle. Out of 200 sent, one undelivered email is 0.5%; out of 500, it's 0.2%. The percentage settles quickly as more mail goes out.

This is why an early bounce-rate number should be read gently. To make that clear, your dashboard labels early figures as "still building data" — its way of saying "there aren't enough emails yet for this percentage to mean much, so don't read too much into it." Once a healthy volume has been sent, the number becomes meaningful and, in almost every case, drops right down.

Temporary delays don't count against you

There's a second reason a bounce rate can briefly look higher than it should — and this one WarmySender quietly handles for you. Sometimes a receiving mail server is momentarily unavailable and briefly delays a message. That's a temporary hiccup on the other side — the recipient's provider being busy for a moment — not a problem with your mailbox at all. It's the email equivalent of a shop being closed for five minutes; the delivery simply tries again shortly after.

Because these blips aren't real failures, WarmySender does not count temporary receiver-side delays against your bounce rate or your health score. Only genuine, permanent delivery failures — where a message truly can't be delivered — count toward the number you see. So a passing slowdown on the receiving end never inflates your bounce rate, and there's nothing for you to do when one happens.

You can read more about these harmless, self-correcting moments in why an inbox shows "temporary provider slowdown".

What a genuinely high bounce rate can point to

Once you've sent a healthy volume and the number has had a chance to settle, a bounce rate that stays well above a few percent is worth a quick look. It usually points to something simple in the mailbox's setup rather than anything complicated. Run through this short checklist:

• Check the mailbox address is exactly right. A small typo in the email address — a missing letter or a transposed character — is the most common cause of real bounces. Confirm the address on the mailbox matches the one you actually sign in to.
• Make sure your email authentication is set up. Email authentication — the SPF, DKIM, and DMARC records on your sending domain — tells receiving inboxes that your mail is genuinely from you. Without it, more of your mail can be refused. How warmup protects your deliverability explains why this matters and what good looks like.
• Give a brand-new domain time to settle. A domain registered very recently, before its records have been put in place, can see more bounces simply because receiving inboxes don't recognise it yet. Adding your authentication records and letting the domain establish itself resolves this.
• Watch for a typo in the domain itself. If the part after the "@" was entered with a slip (for example a missing letter in the company name), mail to it can't be delivered. Double-check the domain is spelled correctly on the mailbox.

If your domain has been flagged somewhere rather than mis-set-up, my sending domain shows on a blocklist covers that separately.

When to worry — and when not to

Here's the simple rule of thumb, so you always know which situation you're in:

• Not worth worrying about: a high-looking percentage while the count is small and labelled "still building data", or the occasional brief blip from a temporary receiver-side delay. Both are normal, both settle on their own, and no action is needed. The best thing you can do is let warmup keep running and check again once more mail has gone out.
• Worth a quick look: a bounce rate that stays well above a few percent after you've sent a healthy volume — that is, once the number is no longer a small sample. That's the signal to run the short checklist above: confirm the address, confirm your email authentication, and make sure the domain is spelled correctly and established.

In practice, the vast majority of "high bounce rate" moments fall into the first group and clear themselves as your warmup volume builds. A genuinely high rate is uncommon, and when it does appear it's almost always a small setup detail you can fix in a couple of minutes.

Frequently asked questions

Why does my warmup show a high bounce rate with only a few emails sent?

Because bounce rate is a percentage, and percentages swing wildly when the total is small. In your first days only a handful of warmup emails have been sent, so a single undelivered email turns into a big-looking figure — 1 undelivered out of 16 sent shows as 6.3%, even though that's really just one email. As your daily volume grows, that same one-off barely moves the number and it settles quickly. That's why your dashboard labels early figures as "still building data": there simply aren't enough emails yet for the percentage to mean much. It's almost never a sign that anything is wrong with your mailbox.

Do temporary delivery delays count against me?

No. Sometimes a receiving mail server is momentarily unavailable and briefly delays a message — that's a temporary hiccup on the recipient's side, not a failure on yours, and the delivery simply tries again shortly after. WarmySender does not count these temporary receiver-side delays against your bounce rate or your health score. Only genuine, permanent delivery failures — where a message truly can't be delivered — count toward the number you see. So a passing slowdown on the receiving end never inflates your bounce rate, and there's nothing you need to do when one happens.

What bounce rate is normal for warmup?

Once a healthy volume has been sent and the number has settled, a healthy warmup bounce rate is very low — typically well under a few percent, and often close to zero, because warmup mail travels between real, working inboxes. Early on, before much mail has gone out, the percentage can look higher purely because of the small sample, so read it gently until it's out of the "still building data" stage. As a simple guide: a low, settled number is exactly what you want; a number that stays well above a few percent after a healthy volume is the one worth checking.

How do I lower my warmup bounce rate?

First, give it time — in most cases the number drops on its own as your volume grows and the small-sample effect fades, so there's nothing to fix. If it stays well above a few percent after a healthy volume, run a short checklist: confirm the mailbox address is spelled exactly right (a small typo is the most common real cause), make sure your email authentication (SPF, DKIM, and DMARC) is set up on your sending domain, and check the domain itself is spelled correctly and has had time to establish itself if it's brand new. Those few setup details resolve the large majority of genuinely high bounce rates. Temporary receiver-side delays need no action at all, since they don't count against you.

Related reading

• Why warmup shows only a few emails early in the day — Why an early, partial count is normal and fills in by evening
• Why an inbox shows "temporary provider slowdown" — The harmless, self-correcting blips that don't count against you
• How warmup protects your deliverability — Email authentication and the reputation your warmup is building
• How email warmup ramps up — What to expect in your first hour, day, and week as the daily amount grows
• My sending domain shows on a blocklist — What to do if a domain has been flagged
• Your warmup dashboard — Check your bounce rate, send count, and mailbox health
• Full documentation — Cold Emailing, Email Warmup, LinkedIn Outreach, and Multichannel guides