How to Improve Email Deliverability for High-Volume Campaigns

Estimated reading time: 11 minutes

A campaign can show a high delivery rate while thousands of messages sit in spam. The receiving servers accepted the emails, but subscribers may never see them.

The latest benchmark data shows how differently large senders can perform across mailbox providers. Improving email deliverability for large campaigns starts with your audience and sending setup, followed by a schedule you can adjust as delivery results come in.

Key Takeaways

• Senders in the 1M+ monthly volume tier averaged 54.32% inbox placement in Q2 2026, up from 27.30% a year earlier.
• Verify authentication, clean the audience, and apply suppression lists before launch.
• Increase volume gradually and monitor each major mailbox provider separately.
• Make unsubscribing easy and process requests promptly.
• Check inbox placement alongside delivery rates, complaints, and SMTP errors.

What the 2026 Benchmarks Show About Large Senders

The GlockApps Email Deliverability Benchmark Report 2026 vs 2025 compares placement-test results across 17 ESPs, 11 inbox providers, and five monthly sending-volume tiers. Its Q2 figures show that the three largest tiers followed different paths:

Monthly sending volume	Q2 2025	Q2 2026	Change
50K–200K emails	63.29%	64.20%	+0.91 percentage points
200K–1M emails	67.87%	63.22%	−4.65 percentage points
1M+ emails	27.30%	54.32%	+27.02 percentage points

The largest tier nearly doubled its average inbox placement, but still recorded a lower rate than the other two groups. Meanwhile, senders in the 200K–1M tier lost ground.

These figures come from client placement tests, grouped by monthly sending volume. They do not measure the size of an individual campaign or establish why results changed. Use the matching tier as a reference, then compare your own performance by provider and over time.

For planning purposes, the useful lesson is to check whether your current sending program can support the next increase. A larger mailing list or a familiar ESP does not establish that on its own.

1. Build the Campaign Audience Before Setting the Volume

Start with the subscribers who should receive this particular message. A database of 500,000 contacts does not necessarily give you an audience of 500,000 for every promotion.

Before exporting or scheduling the campaign:

• Apply suppression lists. Exclude unsubscribed contacts, known spam complainants, and addresses that have permanently failed because the mailbox does not exist.
• Remove duplicates. Check for overlap between segments and scheduled automations.
• Review subscriber sources. Investigate imports with unclear consent or unusually high bounce and complaint rates.
• Separate inactive contacts. Keep subscribers with no recent meaningful activity out of the main promotional send.
• Match the offer to the audience. Use stated preferences, purchase history, or relevant product interest to decide who receives it.

Email verification can help identify invalid addresses. It cannot establish permission or prove that a person still wants your messages.

Choose an inactivity window that fits your normal sending frequency and buying cycle. A weekly newsletter and an annual renewal reminder need different rules.

Avoid relying entirely on opens. Apple Mail Privacy Protection can distort open tracking, and automated security tools can generate clicks. Use filtered engagement data where available, along with purchases, replies, and other relevant customer activity.

2. Check Authentication on a Real Campaign Message

Publishing DNS records is only part of the job. Send a message through the platform and configuration you will use for the campaign, then inspect its authentication results.

Confirm that:

• SPF passes for the envelope sender domain used by your sending service.
• DKIM passes with the expected signing domain.
• DMARC passes, with at least one passing SPF or DKIM domain aligned with the visible From domain.
• Forward and reverse DNS are valid for the sending infrastructure.
• TLS is enabled for transmission.

Ask your email service provider to confirm infrastructure settings it manages for you.

Major mailbox providers have additional requirements for bulk traffic:

Mailbox provider	Bulk sender requirements to account for
Gmail	Bulk classification begins around 5,000 messages to personal Gmail accounts within 24 hours. SPF, DKIM, and DMARC are required, along with other sender requirements.
Yahoo	Bulk senders must use SPF and DKIM, publish DMARC, and support easy unsubscribing.
Outlook.com	Domains sending more than 5,000 messages daily to its consumer service must meet SPF, DKIM, and DMARC requirements.

A DMARC policy of p=none meets these providers’ minimum policy requirement. It does not guarantee inbox placement.

Gmail counts messages from the same primary domain together, including subdomains. Splitting a campaign across subdomains does not remove bulk sender obligations.

3. Choose Infrastructure That Fits Your Sending Pattern

A large campaign does not automatically require a dedicated IP.

A dedicated IP gives you control over its sending reputation, but it also requires enough regular traffic to establish and maintain that reputation. A single large promotion followed by weeks of silence can be a poor fit for a manually managed dedicated IP.

Discuss these options with your ESP:

• Irregular campaigns: A well-managed shared pool or a managed dedicated IP service.
• Consistent, substantial volume: Dedicated IPs with a planned warmup and ongoing monitoring.
• Large promotional and transactional streams: Separate sending infrastructure, with distinct IP pools where appropriate.

Keep promotional campaigns separate from password resets, receipts, and other time-sensitive messages. Separate subdomains and DKIM identities help organize those streams; separate IP pools can also reduce shared IP reputation exposure.

Subdomains do not provide complete reputation isolation. Mailbox providers may still associate them with the same organization, so poor practices in one stream can remain a wider concern.

The report also shows why ESP averages need context. Amazon SES rose from 44.49% in H1 2025 to 54.84% in H1 2026, a gain of 10.35 percentage points. Mailgun fell from 40.69% to 33.20%. Those figures describe the tested traffic; they do not establish how your campaign would perform after a platform switch.

Before migrating, investigate authentication, list quality, and provider-specific failures in your current setup. If a move is necessary, complete the migration and warmup before the major promotion.

4. Increase Volume in Stages

Compare the planned campaign with what you normally send to each provider.

Suppose your program usually sends 10,000 messages a day, but the next promotion targets 100,000 subscribers. Scheduling all 100,000 for the same morning introduces a sharp change in traffic. An established domain can still encounter rate limiting when its sending pattern changes abruptly.

Build the schedule around your existing history:

1. Start with a relevant segment of recently engaged subscribers.
2. Review acceptance, deferrals, bounces, and available complaint data.
3. Increase volume when results remain stable.
4. Slow or pause the affected provider’s traffic if delivery deteriorates.
5. Expand the audience only while performance supports it.

There is no universal daily increase that makes every campaign safe. The right pace depends on your infrastructure, previous volume, audience quality, and mailbox provider responses.

Plan volume separately for Gmail, Yahoo, Outlook, and other major destinations. An IP with a history of sending to Gmail may have little established traffic with Outlook.

Also check what is already running. Welcome sequences, abandoned-cart reminders, newsletters, and promotional resends all contribute to the day’s volume and subscriber frequency. Your campaign calendar should account for that overlap.

5. Make Unsubscribing Work Before the Send

A large campaign can generate opt-outs within minutes. Those requests need to reach your suppression system before another campaign or automation sends to the same people.

For bulk marketing messages, Gmail and Yahoo require one-click unsubscribe support. This is implemented through email headers; a footer link alone does not satisfy that requirement.

Check that:

• The message includes correctly configured List-Unsubscribe and List-Unsubscribe-Post headers.
• The one-click endpoint processes the request without a login or confirmation page.
• A visible unsubscribe link appears in the email body.
• Suppression updates reach the relevant campaigns and automations.
• A later contact import cannot silently restore an unsubscribed address.

Yahoo requires unsubscribe requests to be honored within two days. Google recommends processing them within 48 hours. Aim to apply them immediately wherever your platform supports it.

Test the complete process with a subscribed test contact. Confirm that the contact is actually excluded from the next relevant send.

6. Test the Final Email Through Your Sending Platform

Use the finished campaign for testing, including its subject line, HTML, images, tracking links, and unsubscribe settings. Send it through the same infrastructure planned for launch.

A forwarded copy or a message sent from an employee’s mailbox will not reproduce the campaign’s authentication and sending path.

Before releasing the full campaign, run the message through GlockApps Inbox Insight to check placement across mailbox providers and review authentication, spam filter, and blocklist findings. Investigate provider-specific problems while you can still adjust the send.

Review the message itself, too:

• Does the sender name clearly identify your business?
• Does the subject line accurately describe the offer?
• Do links and redirects reach the intended destinations?
• Does the message remain understandable when images are blocked?
• Do personalized fields have sensible fallback text?
• Can subscribers read the offer and find the unsubscribe link on mobile?

Treat seed-test results as diagnostic evidence. They show how test mailboxes handled the message; they cannot predict placement for every subscriber or reproduce the effects of sending the entire campaign.

7. Monitor Each Mailbox Provider Separately

A campaign-wide average can hide a problem affecting one part of your audience. The report’s 1M+ monthly volume tier makes that clear:

Inbox provider	Q2 2025	Q2 2026
Amazon WorkMail	46.10%	94.45%
Gmail, accounts aged 6+ months	31.50%	68.04%
Outlook	14.09%	35.97%
Google Workspace	32.54%	18.26%

Inbox Placement for the 1M+ Monthly Volume Tier: Selected Providers

Google Workspace moved in the opposite direction from the other providers shown, declining by 14.28 percentage points. Stronger results at personal Gmail accounts would have told you little about placement at Workspace accounts in this sample.

Keep personal Gmail and Google Workspace separate in your reporting. Apply the same distinction to Outlook.com consumer accounts and Microsoft 365 business mailboxes. Then investigate changes against your own baseline:

• More temporary SMTP failures: Check rate limits, authentication, and the receiving server’s explanation.
• More invalid-address bounces: Review list age, acquisition sources, and recent imports.
• Rising complaints: Examine audience selection, frequency, and whether recipients recognize the sender.
• Lower inbox placement: Compare recent changes to the sending setup and message.
• Lower clicks or conversions: Check placement alongside audience fatigue and offer relevance.

Google recommends keeping the spam rate shown in Postmaster Tools below 0.1% and avoiding 0.3% or higher. Yahoo requires bulk senders to remain below 0.3%.

These are provider-specific measurements. A complaint rate displayed by your ESP may use different data and a different denominator. Read each metric’s definition before comparing it with a mailbox provider’s threshold.

Use live delivery logs during the send, then review provider dashboards as their data updates. Google Postmaster Tools data is updated daily, so it cannot serve as an immediate stop signal for every batch.

8. Decide in Advance When to Pause

Assign someone to monitor the campaign and make sure they can stop or slow it. Agree on escalation points using your usual performance, your ESP’s limits, and provider requirements.

When something changes, respond to the actual failure:

• Temporary 4xx responses: Check the accompanying explanation. Let your sending system retry with appropriate delays; reduce traffic when the provider indicates throttling.
• Permanent invalid-address failures: Suppress the affected addresses.
• Authentication or policy rejections: Pause the affected stream and fix the stated cause. A rejection does not always mean the recipient address is invalid.
• Complaint increases: Stop expanding the audience and review the segment, acquisition source, and recent sending frequency.
• A growing delivery queue: Check whether delayed messages will still be relevant when they arrive, especially for short promotions.

Avoid manually resending messages that remain queued for automatic retry. You can create duplicates and add traffic while the original delivery problem is unresolved.

Keep a record of changes made during the campaign. If you alter the audience, sending rate, and template together, identifying what helped becomes much harder.

Conclusion

The 2026 benchmarks show how much inbox placement can vary across sending volumes and mailbox providers. Keep your audience relevant, verify authentication, and increase volume based on your own delivery results. Before your next large campaign, run a GlockApps inbox placement test to spot provider-specific issues while there’s still time to address them.

FAQ