Today, ensuring your messages land in the inbox (not the spam folder) is considered a crucial business aspect. Email marketers, sales teams, and businesses rely on high email deliverability to maximize engagement, build trust, and drive conversions. However, reaching the inbox consistently is no easy feat, especially when dealing with complex email filtering systems like Microsoft 365 (MS365).
Why seed lists matter in email deliverability
Using seed lists is a widely used strategy to gauge and improve email deliverability. These are basically lists consisting of email addresses distributed across different mailbox providers (e.g., Outlook, Gmail, Yahoo) to help with warming up domains and improving sender reputation before launching large-scale email campaigns.
But the question remains—do all seed lists perform equally well? And more importantly, does the type of seed list used impact MS365 deliverability?
The role of seed lists in email deliverability
Seed lists serve a crucial role in improving email performance. See, even if these addresses are not real customers, they can be used to warm up the sender’s domain before engaging in a full on campaign. That way, senders’ domains are already deemed as trustworthy by various ISPs and email providers. When they execute their large-scale outreach campaigns, their emails have a larger chance of landing in the inbox.
However, not all seed lists are created equal. But understanding their impact can mean the difference between a successful email campaign and one that fails due to poor inbox placement.
The importance of MS365 inbox placement for senders
Microsoft 365 (MS365) has one of the most sophisticated spam filters among major email service providers. In our recent research, this was actually a key finding. You can read more about it here: Inbox vs. Spam: How DNS Configuration & Email Templates Affect Your Email Success.
Businesses using Outlook are subject to strict filtering rules, which can make inbox placement unpredictable, especially for bulk email senders. Plus, even if emails do pass authentication checks (SPF, DKIM, DMARC), MS365 still applies additional behavioral analysis to assess sender reputation, engagement, and message content.
For businesses that rely on MS365 to communicate with clients, partners, or prospects, landing in the inbox is non-negotiable. A poor deliverability rate can lead to missed opportunities, damage to the brand reputation, and reduced customer engagement.
This is why optimizing email deliverability for MS365 is a pressing concern. With Warmy.io’s latest research, we now have clearer insights into how seed lists affect inbox placement and sender reputation.
How Warmy.io’s research brings clarity to bulk email strategies
In this research, we analyzed:
- The effectiveness of new vs. established seed lists in improving MS365 deliverability.
- The impact of custom SMTP mailboxes with bad deliverability on inbox placement rates.
- How bulk sending volume influences filtering risks.
Our findings reveal game-changing insights that can help email senders maximize their deliverability and avoid costly mistakes.
The experiment: Comparing new and established seed lists
To determine whether using a new or established seed list improves email deliverability to Microsoft 365 (MS365), we conducted a structured experiment with five different email sending flows. Each flow was designed to test specific variables affecting inbox placement, sender reputation, and bulk sending strategies.
New vs. established seed lists: what’s the difference?
The new seed lists refer to a seed list that hasn’t been used before. This seed list is provided by Warmy following our advanced strategies. Meanwhile, the established seed lists have already been used for some time.
This experiment aimed to discover and pinpoint a difference between the two seed lists which were created using different logic, algorithm, and technology.
What we tested: the five flows explained
Each flow represented a different email sending scenario, allowing us to observe how different combinations of seed list types and sender reputation influenced deliverability over time.
1. Flow 1: Control group (baseline deliverability)
The control group basically establishes a baseline deliverability using sender addresses with already known reputations. The purpose of this control group is a benchmark to compare the results of the other flows with.
2. Flow 2: Bulk sending with established seed list
This group used a bulk sending strategy to 175 Outlook mailboxes using Warmy’s established (classic) seed list.
3. Flow 3: Bulk sending with new seed list
In this group, emails were sent to 175 Outlook mailboxes from Warmy’s new seed list.
4. Flow 4: Custom SMTP mailboxes with bad deliverability + established seed list
This flow tested inbox placement by using custom SMTP mailboxes that had a history of poor deliverability to send emails to an established seed list. This will determine if using blocked SMTP mailboxes affects deliverability when combined with an external seed list.
5. Flow 5: Custom SMTP mailboxes with bad deliverability + new seed list
In this flow, inbox placement was tested using custom SMTP mailboxes with bad deliverability, using a new seed list this time. The purpose is to test whether a new seed list would have an effect on SMTP mailboxes with poor deliverability compared to an established seed list.
Senders and receivers
For senders, flow 1-3 used custom SMTP addresses, while Flows 4-5 used custom SMTP mailboxes with bad deliverability. The receivers were all Outlook emails in the respective seed lists.
Testing factors: what influences inbox placement?
Throughout the experiment, we analyzed several key factors that directly impact email deliverability:
- Inbox vs. spam folder placement rates: These metrics show the percentage of emails landing in the primary inbox vs. those being filtered into the spam folder. Higher spam placements mean poor sender reputation or stricter filtering by MS365.
- Bounce rates: This refers to the percentage of emails that are rejected by the recipient’s mail server and never delivered. This metric matters because high bounce rates damage sender reputation, which leads to stricter filtering and increased likelihood of future emails going to spam.
- The impact of sender reputation: Sender reputation is determined by various factors—past email behavior, engagement rates, and spam complaints. Flows 4 and 5 specifically were designed to uncover how a bad sender reputation can affect inbox placement.
- Deliverability fluctuations over time: Email deliverability is not static—it changes over time due to sender behavior, reputation shifts, and Microsoft’s evolving filtering algorithms.These are important to monitor as sudden deliverability drops can be caused by factors like sending spikes, domain reputation issues, or engagement signals from recipients. Monitoring deliverability gives senders the information they need to have to make informed decisions.
Key findings: What the research revealed
Here is a summary of data observed in each flow:
We also generated detailed and comparative charts of MS365 placement rates over time to highlight the differences across flows and between Flows 4 and 5 in particular.
Now, on to the key findings…
Key finding #1: New seed lists outperformed established seed lists
One of the most significant discoveries from the research was that new seed lists delivered better long-term results than established seed lists. Specific observations include:
- While both types of seed lists showed strong initial inbox placement rates, the new seed list demonstrated better stability and faster recovery from deliverability dips.
- The established seed list actually had a higher starting deliverability, but this was likely due to a better initial sender reputation—and not the effectiveness of the seed list itself.
- The new seed list showed greater stability over time, meaning there were fewer fluctuations and a steadier deliverability rate. The established seed list in Flow 4 showed some sporadic movements as it reached 100% deliverability at times, while also dropping unpredictably at later stages. Flow 5, with the new seed list, maintained good deliverability for longer periods—staying at 100% without any performance dips later on.
Key finding #2: Custom SMTP mailboxes with poor deliverability inflicts further damage
Flow 4 and Flow 5 used custom SMTP mailboxes with bad deliverability. This resulted in unpredictable inbox placement trends for both flows. This means that SMTP mailboxes with a history of poor sender reputation are not recommended for bulk sending—as this can lead to highly unstable and erratic inbox placement rates.
We did observe the following differences between the two flows:
- Flow 5 with the new seed list recovered and improved faster, by 50% in just five days.
- Flow 4, on the other hand, improved by only 30% within the same period.
While these numbers showed a difference when it comes to using established vs. new seed lists, continuing to use custom SMTPs with bad deliverability can create severe deliverability issues.
Key finding #3: Lower email volumes improve deliverability
One of the most consistent findings in email deliverability research is that sending volume matters. In fact, sometimes, the sending volume can trigger specific SMTP errors.
This study reinforced that lower email volumes lead to better inbox placement, while excessive volumes increase filtering risks and data fluctuations. Thus, bulk email sending should be carefully managed.
The verdict: New vs. established seed lists
In a nutshell, established seed lists showed a strong start but unstable performance in the long run. Though it had a high inbox placement, the results fluctuated. Meanwhile, the new seed lists had a relatively slower start, yet a more reliable growth over time.
Warmy’s research team’s final verdict? Warmy’s new seed lists are the smarter choice due to better long-term inbox placement stability and faster recovery from poor deliverability.
Best practices for maximizing MS365 deliverability
Improving email deliverability to Microsoft 365 (MS365) requires strategic planning, continuous monitoring, and the right tools. Implementing proven best practices can also significantly improve inbox placement rates while avoiding common deliverability pitfalls.
Avoid bulk sending pitfalls
Sending large volumes of emails too quickly triggers spam filters and reduces inbox placement rates. MS365 is particularly sensitive to sudden sending spikes, meaning senders must gradually scale up their email volume to avoid being flagged as suspicious.
- Start slow, then scale up. A gradual warmup period helps establish trust with Microsoft’s filtering system.
- Monitor daily sending limits: MS365 applies stricter filtering to high-volume senders, so maintaining a steady and moderate email volume is key.
- Segment email lists: Instead of blasting the same message to thousands of recipients at once, segment your list and distribute sends over time.
Recommendation: Use Warmy.io’s automated warmup process to gradually increase email sending volumes in a way that aligns with MS365’s filtering algorithms. This prevents sudden spikes that can negatively impact sender reputation.
Use Warmy’s advanced seed lists
Warmy.io’s advanced seed lists are designed to generate successful deliverability rates. Unlike traditional seed lists, Warmy’s advanced new seed lists are genuine email addresses that also simulate real user interactions.
For example, your emails are not only opened—they are scrolled through, clicked on, and marked as important. If an email lands in the spam folder, it is manually removed from the spam and marked as important—sending a message to ISPs that your domain is to be trusted.
Conclusion: Key takeaways for email senders
Our study confirms that internal seed lists consistently outperform external ones when it comes to long-term inbox placement and deliverability stability.
Over time, internal seed lists demonstrated:
- Greater stability with fewer fluctuations
- Stronger long-term deliverability performance
- Faster recovery from inbox placement issues
For email senders aiming to build and maintain a strong sender reputation, leveraging internal seed lists is a smarter, more sustainable approach.
Another critical takeaway is using blocked or poorly rated SMTP mailboxes severely damages email deliverability. Our research showed that flows using custom SMTPs with bad deliverability struggled with erratic inbox placement and unstable performance. Even when paired with an internal seed list, recovery was limited.
The study confirmed that email volume plays a significant role in inbox placement. Excessive bulk sending triggers spam filters, while moderate and controlled sending improves deliverability.
Final takeaway: The smartest strategy for MS365 email deliverability
- If your goal is long-term email deliverability success, here’s what you should prioritize:
- Use an internal seed list for better inbox placement stability and faster recovery.
- Avoid blocked or poorly rated SMTP mailboxes—they lead to severe reputation damage.
- Control your sending volume to optimize deliverability without triggering spam filters.
Warmy.io is the ultimate email warmup and deliverability solution. Combining automated email warmup with advanced seed list features and capabilities, Warmy helps businesses take their emails to the next level—landing in inboxes consistently and avoiding the spam folder.
