Based on aggregate Mailblast campaign data, list size has a counterintuitive relationship with deliverability: campaigns sending to fewer than 500 recipients average a 52.93% open rate but a 1.87% bounce rate, while campaigns sending to 250,000-1,000,000 recipients average just a 4.89% open rate and a 0.08% bounce rate. Small is not better; it is just different.
This post walks through the full per-bucket data, what is actually going on, and what it means for choosing list size for an Amazon SES marketing program.
How List Size Actually Affects Bounce, Open, and Complaint Rates
Bounce rate drops sharply as list size grows. Campaigns sending to fewer than 500 recipients average a 1.87% aggregate total bounce rate; campaigns sending to 50,000-250,000 recipients average 0.09%. That is roughly a 20-fold reduction across the size range. Every tier stays well below the 5% bounce rate that puts an Amazon SES account under review (AWS, 2026), but the small-list tier is the only one anywhere near the warning band.
Open rate moves in the opposite direction. Sub-500 campaigns average 52.93%; 10k-50k campaigns average 21.31%; 250k-1M campaigns average 4.89%. The reason is not deliverability - it is audience tightness. Small lists are typically engaged niche audiences, founder newsletters, and beta-user announcements. Large lists are typically broad consumer marketing where most recipients are dormant.
| List size bucket | Total bounce rate | Hard bounce rate | Open rate | Complaint rate | Unsub rate |
|---|---|---|---|---|---|
| <500 recipients | 1.87% | 0.50% | 52.93% | 0.025% | 0.66% |
| 500-2,500 | 0.44% | 0.08% | 45.51% | 0.012% | 0.28% |
| 2,500-10,000 | 0.64% | 0.23% | 29.96% | 0.010% | 0.16% |
| 10,000-50,000 | 0.12% | 0.01% | 21.31% | 0.007% | 0.08% |
| 50,000-250,000 | 0.09% | 0.01% | 16.41% | 0.008% | 0.08% |
| 250,000-1,000,000 | 0.08% | 0.02% | 4.89% | 0.006% | 0.09% |
The complaint rate is the one number that stays remarkably stable across all size buckets. Every tier sits in the 0.006-0.025% range, far below the 0.1% complaint rate that puts an Amazon SES account under review. That stability is itself a useful signal: complaints reflect recipient sentiment about your content, which does not change much with list size. Bounce rate reflects list hygiene, which changes a lot.
Why Small Lists Bounce More
The 1.87% bounce rate for sub-500 campaigns is dominated by three sender patterns: occasional newsletters with stale addresses, test campaigns to imported contact lists, and one-off broadcasts to old subscriber bases that have not been re-engaged. Small-volume senders typically do not run a regular hygiene process because the per-campaign cost of doing so feels disproportionate to the size.
The hard-bounce numbers tell the same story more sharply: 0.50% for sub-500 campaigns dropping to 0.01% at the 10k-250k tiers, a roughly 50-fold reduction. The selection is operational. Senders running campaigns at 50,000+ recipients have list hygiene processes because Amazon SES will throttle them otherwise; senders running at 500 typically do not.
The Open Rate Inversion: Smaller Audience, Higher Engagement
Open rates run inverse to bounce rates: 52.93% for sub-500 campaigns, 21.31% for the 10k-50k tier, and 4.89% for the 250k-1M tier. Three drivers account for the pattern.
First, audience composition changes with size. A 500-person list is typically a tight niche - paying customers, early-adopter beta users, or a founder's personal network. These people open emails because they want what's inside. A 250,000-person list is usually a broad consumer or B2B marketing list with a long tail of dormant subscribers.
Second, send frequency correlates with size. Small newsletters tend to send weekly or monthly; large broadcast lists send daily or several times a week. Higher frequency erodes per-message engagement because recipients tune out.
Third, content type correlates with size. Small lists tend to skew toward personalized or curated content (founder updates, expert analysis); large lists tend toward broad promotional content. The personalization gap shows up directly in opens.
The practical implication: an open rate of 50%+ on a 500-person list and an open rate of 5% on a 250k-person list can both be "healthy" outcomes. Compare to peer campaigns at the same size tier, not to a universal "good open rate" number.
What This Means for Choosing List Size
The data doesn't say "make your list bigger" or "make it smaller." It says list size is a poor proxy for list quality, and the metrics that matter (deliverability, engagement, complaint rate) follow operator behavior more than they follow audience headcount.
Three practical takeaways:
For early-stage senders growing a list to 10,000+: the deliverability inflection happens around the 10,000-recipient mark. Bounce rates drop sharply once your operations are running list hygiene at that scale. Plan for the operational discipline before you hit 10k.
For mid-sized operators (10,000-250,000): you are in the sweet spot of the data. Bounce around 0.09-0.12%, complaint around 0.007-0.008%, open around 16-21%. If your numbers are materially worse than these benchmarks at this scale, the problem is list quality or authentication, not list size.
For large-list operators (250,000+): open rate around 5% isn't a problem; it's the size tier's baseline. Focus on bounce rate (target under 0.1%), complaint rate (target under 0.01%), and unsub rate (target under 0.1%). These three metrics determine whether AWS keeps trusting your reputation.
Quarter-over-quarter bounce-rate spikes are a useful warning sign for this conversation - see our bounce rate benchmarks for what a healthy range looks like. List hygiene events can move account-level metrics meaningfully even at large scale. See why emails go to spam for the broader picture of what drives bounce and inbox-placement outcomes.
FAQ
Do smaller email lists have better deliverability?
No - the opposite is true at scale. Based on aggregate Mailblast campaign data, campaigns sending to fewer than 500 recipients have a 1.87% total bounce rate; campaigns sending to 50,000-250,000 recipients have a 0.09% bounce rate. Smaller campaigns look better on open rates (53% vs 16%) but are roughly twenty times dirtier on bounce.
What's a good open rate for Amazon SES?
Open rate varies dramatically by list size. Across Amazon SES marketing campaigns via Mailblast, campaigns under 500 recipients average 53% open rate; campaigns at 10,000-50,000 recipients average 21%; campaigns at 250,000-1,000,000 recipients average 5%. The shift reflects audience tightness more than deliverability quality - smaller lists tend to be highly engaged subsets, while bigger lists include the long tail.
Why do small lists have high bounce rates?
Small lists are dominated by test sends, infrequent newsletters, hobbyist projects, and lists that have not been maintained recently. They include addresses that have never been validated against bounces. As list size grows, senders typically run more disciplined hygiene processes because at scale the financial and reputation cost of bounces is much higher.
Does sending to a larger list damage deliverability?
Not by itself. The data shows the opposite: aggregate bounce rates drop as list size grows. What matters is who is on the list, not how many. A 10,000-recipient list of engaged subscribers will outperform a 500-recipient list of stale addresses. List quality and recency dominate over list size for deliverability outcomes.
What list size should I target for Amazon SES?
There is no single right answer. The data shows that lists in the 10,000-250,000 range produce the most consistent deliverability metrics: bounce around 0.09-0.12%, complaint around 0.007-0.008%, open around 16-21%. Below 1,000 recipients you are in the noisy zone of small-sender behavior; above 250,000 recipients you are typically running broadcast lists with lower engagement but stable bounce metrics.
How Mailblast Reports List-Size Metrics
Mailblast operates on your own Amazon SES account (BYO-SES, your own AWS keys, your own sending reputation), so the volumes in your dashboard reflect what your SES account actually sent, not a pooled hosted-ESP number. The campaign-level reports include the same per-bucket metrics analyzed here: total and hard bounce, open, complaint, and unsubscribe rates, with the recipient count for each send. That makes it possible to compare your own results against benchmarks like the ones in this post at the same level of granularity.
Methodology: aggregate metrics by per-campaign recipient count across all non-suspended Mailblast Amazon SES sends from 2023-01-01 to 2026-06-10. We excluded accounts with aggregate bounce rates above 3% from this analysis, as these accounts are in active SES reputation trouble and not representative of standard deliverability. Volume-weighted figures use sum of bounces divided by sum of recipients in each bucket. AWS reputation thresholds quoted from official AWS SES documentation.
Disclosure: Mailblast is a hosted management layer for your own Amazon SES account. The benchmarks above are aggregate metrics across Mailblast customers' SES sends; individual results depend on list hygiene, content, and authentication setup on your own AWS account.