Does fluctuating hourly email send volume within a large campaign impact deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 13 Aug 2025
Updated 18 Aug 2025
6 min read
Many marketing teams grapple with the nuances of email sending volume, particularly within large-scale campaigns. There's often a lingering question about whether minor hourly fluctuations, like adding a smaller send in the middle of a massive daily rollout, can negatively impact your email deliverability.
While maintaining a consistent sending pattern is a foundational principle for good email deliverability, the specifics of intra-day fluctuations often get overcomplicated. I often see concerns that are rooted in misunderstandings or outdated information, rather than current best practices.
The fundamentals of volume and reputation
Internet Service Providers (ISPs) and mailbox providers, such as Google and Yahoo, rely heavily on consistent sending patterns to build trust and assign a positive sender reputation. They observe your sending behavior over days, weeks, and even months. A steady volume demonstrates legitimate sending practices and predictability.
Your sender reputation is essentially your trustworthiness score. It determines whether your emails land in the inbox, spam folder, or are rejected outright. This reputation is influenced by various factors, including engagement rates, spam complaints, bounce rates, and, significantly, your sending volume history.
The key concern for ISPs isn't minor hourly variance but rather sudden, drastic changes in volume. For example, a sharp increase in sending volume without proper IP warming can look suspicious and trigger spam filters. Similarly, long periods of inactivity followed by a massive send can also harm your standing. You can find more details on how email volume and fluctuations impact deliverability.
Hourly fluctuations in large campaigns
When you're managing a large campaign, perhaps sending hundreds of thousands or even millions of emails over a day, the idea that a small additional send of a few thousand emails in the middle of that flow could cause issues is generally unfounded. Mailbox providers primarily look at larger trends (daily, weekly, monthly volume) and overall sender behavior. They don't typically scrutinize hourly micro-fluctuations to that degree, especially for established senders.
The receiving systems (MX records) at ISPs don't have a granular view of your internal hourly pacing across all recipient domains. They only see the volume of mail being sent to them at any given time. If your overall sending infrastructure is capable of handling a 1 million email per day volume, adding an extra 5,000 or 10,000 emails during an already active hour is unlikely to make a discernible negative impact on your deliverability.
This scenario differs significantly from warming up a new IP address or domain. During initial IP warming, strict adherence to a gradual, consistent increase in volume is critical to build trust. However, for established senders with a consistent history, the system is already accustomed to high volumes.
Best practices for high volume sending
Consistency is key: Maintain a predictable daily or weekly sending rhythm.
Warm up new IPs: Gradually increase volume for new IPs or domains to build reputation.
Monitor reputation: Regularly check your sender and domain reputation.
Segment intelligently: Send highly engaged content to your most active users.
Beyond hourly variations
Instead of obsessing over precise hourly distribution within a large, ongoing campaign, I encourage focusing on the core elements that truly dictate deliverability outcomes. These include maintaining a clean email list, ensuring your content is relevant and engaging, and swiftly addressing any bounces or spam complaints. A healthy list and positive engagement signals will always outweigh minor hourly volume shifts.
Sudden, large spikes in email volume can indeed trigger spam filters, especially if they deviate significantly from your established sending patterns. This is why major changes in your sending strategy, like a sudden massive increase in daily volume, should be planned carefully with proper warm-up procedures, as outlined by Adobe's guidelines to avoid sudden changes in send volume. Understanding the impact of increasing or decreasing email volume is vital.
While a shared IP environment might see some minor effects from others' sending behavior, on a dedicated IP, you have more control. However, even with a dedicated IP, erratic patterns can pose deliverability risks. The emphasis should remain on overall consistency rather than hourly micromanagement.
Factors influencing deliverability
Engagement: High open rates, clicks, and replies signal legitimate mail.
Reputation: Your sender and domain reputation over time.
List hygiene: Removing inactive users, hard bounces, and spam traps.
Authentication: Proper SPF, DKIM, and DMARC setup.
Content quality: Relevant, personalized, and spam-filter-friendly content.
Less critical factors (for established senders)
Intra-day micro-spikes: Small, transient hourly volume increases within an otherwise stable, large daily send.
Exact hourly tapering: The need to strictly decrease volume throughout the day.
Arbitrary send times: Sending at the top of the hour versus slightly off the hour.
Implementing strategic sending
For large campaigns, focus on ensuring your total daily or weekly volume remains within expected ranges that your sender reputation can support. If you need to send smaller ad-hoc campaigns on the same day as a large broadcast, simply integrate them into your existing sending schedule without fearing minor hourly "bumps". The overall volume and quality metrics are what ISPs predominantly evaluate.
Think of it this way: your email infrastructure and reputation are like a highway. If it's designed for 50,000 cars per hour consistently, adding an extra 5,000 cars for a single hour (totaling 55,000) won't typically cause a traffic jam if the highway is well-maintained and traffic flow is generally good. It's when you suddenly try to push 100,000 cars through, or go from 500 to 50,000 instantly, that problems arise.
Always ensure your email authentication protocols, SPF, DKIM, and DMARC, are correctly configured and aligned. These fundamental layers of security and trust play a far more significant role in inbox placement than granular hourly volume management. Regularly review your DMARC reports for any issues, as they provide valuable insights into your sending patterns and potential authentication failures.
Scenario
Impact on deliverability
Best practice
Consistent, high volume
Very low, provided engagement is good. Builds strong sender reputation.
Maintain consistent volume and monitor engagement.
Minimal impact for established senders. ISPs look at overall patterns.
Focus on daily/weekly consistency, not hourly micro-management.
Infrequent sending
Can lead to slower warm-up for subsequent large sends.
Build up volume gradually when returning from lulls.
Focus on fundamental deliverability principles
The notion that fluctuating hourly email send volume within a large, established campaign significantly impacts deliverability is largely a misconception for the vast majority of senders. While consistency is important, ISPs are more concerned with your overall sending patterns over days or weeks, your sender reputation, and the quality of your recipient list and content.
Instead of overcomplicating hourly schedules, channel your efforts into maintaining a healthy sender reputation, acquiring engaged subscribers, and ensuring robust email authentication. These are the true pillars of successful email deliverability that will keep your messages out of the spam folder and in the inbox.
Views from the trenches
Best practices
Maintain a consistent daily and weekly sending volume, especially for established sender reputations.
Always warm up new IPs or domains gradually before sending high volumes of emails.
Prioritize email list hygiene by regularly cleaning inactive subscribers and hard bounces.
Ensure strong email authentication standards like SPF, DKIM, and DMARC are properly configured.
Focus on delivering engaging and relevant content to maximize positive recipient interactions.
Common pitfalls
Making sudden, drastic increases in email sending volume without a proper warm-up strategy.
Overcomplicating hourly sending schedules within a large, established campaign.
Neglecting engagement metrics like open rates, click-through rates, and complaint rates.
Failing to monitor your sender reputation and address any blocklist (or blacklist) listings promptly.
Sending to unengaged or old lists, which can lead to high bounce rates and spam trap hits.
Expert tips
Consider scheduling sends slightly off the top of the hour to avoid peak congestion at mailbox providers.
If sending smaller, triggered communications, these can consistently run in the background, bridging gaps between bulk sends.
Recipient servers only see the volume directed at them, not your overall hourly send rate across all mailbox providers.
The advice about strict hourly tapering might stem from outdated practices or niche scenarios like low-quality opt-in lists.
For established sending programs, minor hourly fluctuations typically don't trigger spam filters as long as overall daily volume is consistent.
Expert view
Expert from Email Geeks says I've never seen any evidence of intra-day fluctuations mattering all that much. The main thing seems to be volume spikes relative to your typical volume, not hour-to-hour differences. One exception is scheduling messages exactly at the top of the hour; slightly off-hour sends might be better.
2024-07-15 - Email Geeks
Expert view
Expert from Email Geeks says I am deeply skeptical of this guidance. For an IP or domain already warmed up and sending consistently, hourly granularity should not matter. This theory is also nullified by the concept of sending bulk marketing email while triggered communications run in the background. If a 1M send is typical, there shouldn't be risk.
Internet Service Providers (ISPs) and mailbox providers, such as Google and 
Yahoo, rely heavily on consistent sending patterns to build trust and assign a positive sender reputation. They observe your sending behavior over days, weeks, and even months. A steady volume demonstrates legitimate sending practices and predictability.
Adobe's guidelines to avoid sudden changes in send volume. Understanding the impact of increasing or decreasing email volume is vital.
