Does send time impact IP warmup and reputation when using a new ESP?
Michael Ko
Co-founder & CEO, Suped
Published 30 Jun 2025
Updated 16 Aug 2025
6 min read
Starting with a new Email Service Provider (ESP) and warming up a new IP address brings many questions about deliverability. One common concern I hear is about the impact of precise send times on this critical process and on your overall sender reputation. It's a valid question, as consistency is often emphasized in email marketing.
The short answer is that while send time consistency for your audience's engagement matters, the exact hour you send each day generally doesn't directly impact your IP warming or your underlying IP reputation with major mailbox providers. Their algorithms are more concerned with broader patterns and engagement metrics.
Understanding IP warming and reputation
IP warming is the process of gradually increasing the volume of email sent from a new IP address. This helps build a positive sending history and establish trust with Internet Service Providers (ISPs). Without proper warming, a sudden surge of emails from a new IP can trigger spam filters, leading to blocklists (or blacklists) and poor deliverability. A helpful resource on how to warm up a new IP address can provide a good foundation.
The goal of warming an IP address is to demonstrate to mailbox providers that you are a legitimate sender sending desired emails. Mailbox providers like Microsoft and Google monitor various signals, including spam complaints, bounces, engagement rates (opens, clicks), and consistent sending volume. They want to see consistent, positive interactions.
Your sender reputation (or sender score) is essentially a trustworthiness rating assigned by mailbox providers. This score determines whether your emails land in the inbox, spam folder, or are rejected. A new IP address starts with a neutral reputation, so warming is essential to prove you are a good sender and gain their trust. You can find more information about how email reputation transfers during IP warming and resolving deliverability issues.
How mailbox providers assess reputation
Mailbox providers aggregate data over various timeframes, typically daily or hourly, rather than focusing on the precise minute an email is sent. Their systems are designed to handle massive volumes of email, and granular send time within a day is generally not a primary factor in reputation calculations. It's more about the overall daily volume and the quality of engagement it generates.
For instance, if you send 10,000 emails on Monday morning and 10,000 emails on Tuesday afternoon during your warm-up, the shift in exact send time typically won't negatively impact your reputation. What matters more is that both sends generate healthy engagement and low complaint rates, contributing positively to your sender score.
The systems look for patterns over time. Sudden, drastic changes in volume, content, or recipient engagement are what raise red flags, not minor shifts in the time of day. Your sending reputation is built on consistent positive behavior, not strict adherence to a specific minute. This is why it's important to understand why your emails fail to ensure good deliverability.
Practical implications for your send schedule
While the exact send time might not directly influence IP warming, it's crucial for maximizing recipient engagement. Your audience likely has preferred times to interact with emails. Sticking to these optimal times during warm-up can help ensure high open and click rates, which does positively contribute to the sender reputation. A high engagement rate tells mailbox providers that your recipients value your emails.
I often advise clients to consider their historical engagement data when planning warm-up sends. If your subscribers are typically most active in the morning, continue sending in the mornings, even as you ramp up volume. This strategy prioritizes positive engagement signals, which are far more impactful than a rigid time schedule.
Day
Daily volume
Key metrics to monitor
Day 1-3
1,000-5,000
Low bounces, high engagement
Day 4-7
5,000-10,000
Consistent opens/clicks, minimal complaints
Day 8-14
10,000-25,000
Stable deliverability, watch for throttling
Day 15+
Gradual increase to full volume
Maintain positive metrics, monitor for issues
Advanced considerations and common pitfalls
One scenario where send time can indirectly matter is when sending massive volumes. If everyone hits the "send" button at exactly 9:00 AM, mailbox providers experience a huge surge. Staggering your sends by a few minutes past the hour, or distributing them throughout the peak sending window, can sometimes help with throughput and reduce the likelihood of your mail being queued or temporarily deferred. This is less about reputation and more about efficient delivery.
Primary indicators
Engagement: Open rates, click-through rates, and replies.
Spam complaints: Reports from recipients marking your email as junk.
Bounce rates: Especially hard bounces to non-existent addresses.
Volume consistency: Gradual, steady increases during warming.
Content quality: Relevance and absence of spammy characteristics.
Secondary or indirect factors
Exact send hour: Daily reputation calculations are not usually hour-specific.
Minute-by-minute consistency: More about overall daily pattern.
Minor schedule shifts: Small changes within a day are typically ignored.
Timezone differences: ISPs typically normalize or consider global activity.
While your send time itself typically won't land you on a blocklist (or blacklist), other related issues during IP warming can. For instance, sending to unengaged users or invalid addresses can lead to spam traps and high bounce rates, which are significant negative signals. You can learn more about SparkPost's IP warm-up strategy and why it matters for deliverability.
It's crucial to ensure all your email authentication protocols are correctly set up from day one with your new ESP. This includes SPF, DKIM, and DMARC. Even with a perfect send schedule, authentication failures can lead to deliverability problems, especially with major providers. For example, some companies find their emails go to spam when migrating ESPs.
Warning: sudden volume spikes
Avoid sending disproportionately large volumes of email at once, especially during the initial phases of IP warming. A sudden, unexplained surge of email from a new or minimally warmed IP address is a classic spamming pattern and will almost certainly result in your emails being flagged, throttled, or blocklisted by mailbox providers. This will severely damage your new IP's reputation and lead to poor deliverability.
Final thoughts on send time and reputation
In summary, while the exact time you press send each day isn't a primary factor in your IP warming or reputation with a new Email Service Provider, the consistency of your overall sending patterns and the quality of engagement your emails generate are paramount.
Focus on building trust with mailbox providers by gradually increasing volume to your most engaged subscribers, maintaining low complaint rates, and ensuring proper email authentication. These are the true determinants of a healthy sending reputation and successful deliverability. Learn more about DMARC, SPF, and DKIM for robust email security.
Views from the trenches
Best practices
Prioritize sending to your most active and engaged subscribers first during the warm-up period.
Ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured from the start.
Monitor engagement metrics closely, as positive interactions build sender trust and reputation.
Gradually increase sending volume, adhering to a structured and consistent warm-up schedule.
Continuously clean your email list to avoid hitting spam traps and invalid or inactive addresses.
Common pitfalls
Sending large, sudden bursts of email from a new IP address without proper warming.
Failing to segment your list and sending to unengaged or inactive subscribers early on.
Ignoring bounce rates and spam complaint data, which are critical feedback signals.
Neglecting to set up or verify email authentication records before beginning sends.
Assuming that a shared IP eliminates the need for any form of reputation building or monitoring.
Expert tips
Consider staggering your sends by a few minutes past the top of the hour for very large volumes.
Focus on hourly or daily volume consistency rather than minute-level timing for reputation.
Align send times with your audience's historical engagement patterns to maximize positive signals.
Be aware that even with a shared IP, your domain's reputation still needs careful management.
Maintain consistent and high-quality content throughout the entire email warming period.
Expert view
Expert from Email Geeks says that the specific send time should be irrelevant to reputation calculations, as most mailbox providers roll up data to hourly or daily calculations, not minute-by-minute.
2022-03-30 - Email Geeks
Marketer view
Marketer from Email Geeks says they started their first batch in the afternoon but typically send in the mornings because that's what their subscribers are used to.
Microsoft and 
Google monitor various signals, including spam complaints, bounces, engagement rates (opens, clicks), and consistent sending volume. They want to see consistent, positive interactions.
