What is the impact of increasing or decreasing email volume on IP reputation and deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 11 May 2025
Updated 19 Aug 2025
7 min read
Email volume, whether increasing or decreasing, plays a significant role in how mailbox providers perceive your sending practices. It's not just about the content of your emails, but also the consistency and patterns of your sending behavior. Mailbox providers, such as Google and Microsoft, continuously monitor these patterns to determine your IP reputation and, consequently, your email deliverability.
My goal here is to help you navigate the complexities of email volume. We often hear about the dangers of sudden spikes, but understanding the nuances of decreasing volume and the impact of these changes on your IP reputation is equally important for maintaining good standing with internet service providers (ISPs). It’s about building a consistent, trustworthy sending profile.
Maintaining a solid IP reputation is crucial for ensuring your emails reach the inbox rather than the spam folder. Both unexpected increases and decreases in email volume can send signals to mailbox providers that might affect how your messages are handled. It's a delicate dance between sending enough to establish consistency and not so much that you trigger spam filters.
The delicate balance of IP reputation and volume
Mailbox providers maintain complex algorithms to assess the trustworthiness of incoming email. A significant part of this assessment relies on your IP's sending history and behavior. They look for consistency, legitimate engagement, and adherence to established norms.
When you begin sending from a new IP address or significantly alter your email sending patterns, you enter a phase known as IP warming. This is a critical period where you gradually increase your email volume over time to build a positive reputation with mailbox providers. Skipping this step or rushing it can lead to your emails being flagged as suspicious or even blocked. It's how you tell the world, 'Hey, I'm a legitimate sender, and here's my consistent behavior.'
There isn't a single, universal magic formula for acceptable volume fluctuations. Each mailbox provider has its own proprietary algorithms and thresholds. What one provider deems acceptable, another might view with suspicion. This is why a cautious and measured approach is always recommended, especially when you're working with a new or less established IP.
The risk of sudden spikes
Rapid increases in email volume from an IP address that previously sent lower volumes are a major red flag for spam filters. They can signal a potential security breach, spam activity, or a compromised account. This can lead to temporary blocks, throttling, or even blacklisting (or blocklisting, if you prefer) of your IP address.
A damaged IP reputation means your emails are far less likely to reach the inbox. They might be routed to spam folders, rejected outright, or experience significant delays. This directly impacts your campaign performance and can erode trust with your recipients.
The impact of increasing email volume
When you're scaling up your email program or warming a new IP, the rate at which you increase your volume is paramount. I typically advise a gradual approach to avoid triggering spam filters. Sudden spikes make mailbox providers suspicious, as they could indicate a spam attack or a compromised account. It’s better to build trust slowly.
For warmed-up IPs, a good rule of thumb during a planned increase is not to double your daily volume week-by-week. For example, if you sent 100,000 emails per day this week, aim for no more than 200,000 per day next week. This works out to approximately a 10% increase per day for daily senders. This allows mailbox providers to observe your sending behavior and adjust their trust levels accordingly.
Slow ramp-up strategy
Increase email volume incrementally over a period of weeks or months. This helps establish a consistent sending pattern and builds a positive sender reputation with mailbox providers.
Monitor your engagement rates, complaint rates, and bounce rates closely during this period. High engagement and low complaints are positive signals that validate your increased volume.
Engagement metrics are your feedback loop during volume increases. If engagement dips or complaints rise with increased volume, it's a clear signal to slow down or re-evaluate your audience segmentation. Remember, the quality of your recipients matters as much as the quantity of emails you send, especially when warming up an IP address.
The impact of decreasing email volume
While sudden spikes are generally more detrimental, temporary drops in email volume can also have an impact, though usually less severe. If you temporarily reduce your sending, perhaps for a month or so, it's less of an immediate red flag than a sudden surge. However, IP reputation isn't static, it's constantly being evaluated. Therefore, a prolonged decrease can lead to your reputation fading over time.
Based on what I’ve seen, after about 30 days of significantly reduced or no sending, an IP's reputation can begin to diminish. This doesn't mean your IP is permanently damaged, but it might require a mini-warming process when you resume sending at higher volumes. You can't just jump back to your old volume immediately after a long break; it's almost like starting fresh with a slightly tarnished reputation.
The key concern after a temporary decrease is how you choose to re-increase your volume. If you drop significantly and then try to bounce back to your previous peak overnight, it can resemble a sudden spike and trigger the same negative responses from mailbox providers. It's best to re-engage with your most active subscribers first, then slowly expand your reach, mimicking a scaled-down version of your initial IP warming process.
Mailbox provider specific considerations and patterns
One of the most important takeaways about email volume and reputation is that different mailbox providers have different levels of tolerance. What might be acceptable for Gmail may not be for Microsoft (Outlook/Hotmail), and vice-versa. I've observed that some providers, like Google, tend to be more flexible with volume changes, especially if overall engagement and complaint rates remain healthy. Others, notably Microsoft, are known for being much stricter and often cap senders at certain volumes until they build a longer, more consistent history.
It's not just the absolute volume that matters, but also the consistency of your sending patterns. If your business naturally has peaks and troughs, like higher sending during holiday seasons or specific sales events, mailbox providers might recognize and tolerate these fluctuations, provided they are consistent with your historical patterns. However, if you typically send a very consistent daily volume and suddenly triple it without warning, that's where the scrutiny comes in.
Alongside volume, other critical metrics like engagement (opens, clicks), spam complaints, and bounce rates will also be heavily scrutinized during any volume changes. A sudden increase in volume coupled with declining engagement or a spike in complaints will almost certainly harm your IP reputation. Conversely, if you increase volume while maintaining strong engagement, it's a positive signal. This is why monitoring these metrics rigorously is so important.
Views from the trenches
Best practices
Maintain consistent sending patterns to build a reliable IP reputation with mailbox providers.
Gradually increase email volume over time, following an IP warming schedule, especially for new IPs.
Segment your audience and prioritize engaged subscribers for initial sending when increasing volume.
Monitor key email metrics, including open rates, click-through rates, and complaint rates, continuously.
Regularly clean your email lists to remove inactive or invalid addresses, reducing bounces and complaints.
Common pitfalls
Implementing sudden, large spikes in email volume, which often triggers spam filters and blocks.
Attempting to immediately resume previous high sending volumes after a prolonged period of inactivity.
Ignoring engagement metrics during volume changes, leading to declining inbox placement.
Failing to understand the unique volume tolerances of different mailbox providers like Google and Microsoft.
Sending to unengaged or old segments during volume increases, which can inflate complaint rates.
Expert tips
For daily senders, aim for approximately a 10% daily increase in volume during warming phases.
Be aware that IP reputation can start to fade after about 30 days of inactivity.
If you significantly decrease volume, plan a gradual ramp-up when resuming higher sends, even for warmed IPs.
Recognize that consistent sending patterns are often more critical than strict daily volume consistency.
Pay close attention to how Microsoft reacts to your volume; they are generally stricter than Gmail.
Marketer view
Mailbox providers often use unique and proprietary formulas for evaluating email volume and its impact.
2022-11-29 - Email Geeks
Expert view
Volume changes primarily affect deliverability rather than reputation directly, with large spikes indicating potential issues.
2022-11-29 - Email Geeks
Maintaining a healthy sending profile
Navigating the impact of email volume on IP reputation and deliverability requires a nuanced understanding of mailbox provider expectations. The overarching principle is consistency and predictability. Whether you are increasing or decreasing your email volume, the manner in which you do so is more important than the change itself.
For increases, a slow and steady ramp-up, often following a doubling of daily volume week-over-week, is generally the safest approach. This builds trust gradually. For decreases, while less immediately damaging, prolonged reductions can lead to a fading reputation, necessitating a cautious re-warming when volume is resumed. Understanding your audience and their engagement patterns is also crucial to this process. For more on this, read about how email list quality impacts deliverability.
Ultimately, constant vigilance over your sending metrics and an awareness of how different mailbox providers react to your email streams will empower you to maintain a healthy IP reputation and ensure your emails consistently land in the inbox.
Microsoft, continuously monitor these patterns to determine your IP reputation and, consequently, your email deliverability.
Gmail may not be for 
Microsoft (Outlook/Hotmail), and vice-versa. I've observed that some providers, like Google, tend to be more flexible with volume changes, especially if overall engagement and complaint rates remain healthy. Others, notably Microsoft, are known for being much stricter and often cap senders at certain volumes until they build a longer, more consistent history.
