What factors influence email deliverability and how does SenderScore work?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 19 Aug 2025
12 min read
Email deliverability is crucial for any business or individual relying on email for communication, marketing, or outreach. It's not just about sending an email, but ensuring it actually lands in the recipient's inbox, rather than being diverted to a spam folder or blocked entirely. The complexities of getting an email delivered can often be surprising, involving a delicate balance of technical configuration, sender behavior, and recipient engagement. Understanding the various elements that influence whether your messages make it to the inbox is the first step toward achieving consistent email success.
Many factors contribute to this intricate process, ranging from your email infrastructure to the content of your messages. Internet Service Providers (ISPs) and mailbox providers employ sophisticated algorithms to evaluate incoming mail streams, assigning a reputation score to senders. This score, often referred to as sender reputation, is paramount. A good reputation means your emails are trusted, while a poor one can lead to messages being filtered or rejected. This guide will explore the primary factors influencing email deliverability and shed light on how SenderScore works as one measure of your sending reputation.
Sender reputation factors
At the core of email deliverability is sender reputation. This is like a credit score for your email sending practices. Mailbox providers, such as Gmail, Outlook, and Yahoo, continuously monitor various signals to determine if your emails are legitimate and desirable. These signals encompass a wide array of metrics, from the technical setup of your sending infrastructure to the way recipients interact with your messages. Each piece of data contributes to a holistic view of your sending trustworthiness, directly impacting where your emails land, or if they land at all.
A key component of sender reputation is your IP address reputation. This refers to the historical sending behavior associated with the specific IP address (or range of IPs) from which your emails originate. If an IP address has a history of sending spam, hitting spam traps, or generating high complaint rates, emails sent from it are more likely to be blocked or filtered. This holds true whether you're using a dedicated IP or a shared one from an Email Service Provider (ESP).
Similarly, domain reputation tracks the history and trust associated with your sending domain (e.g., yourcompany.com). This reputation is increasingly important, as many mailbox providers prioritize domain-based reputation over IP reputation. Factors like your domain's age, its history of sending legitimate email, and its presence on blacklists (or blocklists) significantly impact its standing. A strong domain reputation signals to ISPs that your emails are from a trusted source, improving your chances of inbox placement. The way you set up your email authentication protocols, like SPF, DKIM, and DMARC, also directly contributes to your domain's perceived trustworthiness and deliverability.
Engagement and content
Mailbox providers closely monitor how recipients interact with your emails. High engagement rates, such as opens, clicks, and replies, signal to ISPs that your content is valuable and desired. Conversely, low engagement, or worse, negative engagement, can severely damage your deliverability. Negative signals include spam complaints, deletions without opening, and moving emails to the trash folder. These actions tell mailbox providers that your emails are either unwanted or irrelevant, leading to lower inbox placement over time. It is important to remember that these metrics are not always a clear indicator of a deliverability problem, but rather a symptom of one. For example, a low open rate might indicate that your emails are not reaching the inbox at all, or that your subject lines are not compelling enough to encourage opens.
Managing your email list effectively is another critical factor. Sending to invalid or inactive addresses leads to high bounce rates, which negatively impact your sender reputation. A healthy list consists of engaged subscribers who have explicitly opted in to receive your emails. Regularly cleaning your list and removing unengaged subscribers or invalid addresses can significantly improve your deliverability. Using a double opt-in process is highly recommended to ensure genuine interest and prevent spam traps from entering your list. Building a clean, engaged list is a foundational step for strong deliverability. Remember, quality over quantity is key when it comes to your email list.
The content and formatting of your emails also play a role. While less impactful than reputation, certain elements can trigger spam filters. Overuse of spammy keywords, excessive images without text, broken links, or suspicious attachments can raise red flags. It's also important to ensure your email's HTML is clean and well-structured. Personalizing your content and providing clear value to your subscribers can boost engagement and reduce the likelihood of being marked as spam. Balance promotional language with genuine value, and always include an easy-to-find unsubscribe link.
How SenderScore works
SenderScore is a service provided by Validity (formerly Return Path) that assigns a rating to outgoing mail server IP addresses. This score, ranging from 0 to 100, is designed to indicate the trustworthiness of your IP. A higher score signifies a better reputation and a higher likelihood of your emails reaching the inbox. It's often compared to a credit score, where a good score demonstrates responsible behavior over time. You can check your score through their online tool.
SenderScore's calculation is based on a 30-day rolling average of several key metrics. These include a variety of sending behaviors and recipient feedback loops. Understanding these components can help you identify areas for improvement. While a high SenderScore is generally positive, it is important to understand that it is just one indicator among many. Mailbox providers use a complex array of internal metrics and proprietary algorithms, so a good SenderScore doesn't guarantee 100% inbox placement.
Here are the primary factors SenderScore considers:
Complaint rates: The percentage of recipients who mark your emails as spam. High complaint rates are a strong negative signal.
Volume: Consistent sending volume is better than erratic spikes or drops. Sudden changes can appear suspicious.
External blocklists (blacklists): Presence on reputable blacklists will significantly lower your score.
Spam trap hits: Emails sent to addresses specifically designed to catch spammers are detrimental.
Rejections: The rate at which mailbox providers reject your emails outright.
Unknown users: The percentage of emails sent to non-existent addresses (hard bounces).
By monitoring these factors, SenderScore provides a valuable snapshot of how your IP address is perceived across the email ecosystem. A consistent, high score indicates healthy sending practices and a positive reputation, which translates to better deliverability.
Improving your deliverability
Positive factors
Email authentication: Properly configured SPF, DKIM, and DMARC records establish your sending legitimacy.
Low bounce rates: A clean email list results in fewer hard and soft bounces.
High engagement: Opens, clicks, and replies indicate recipient interest.
Sender reputation: A history of sending wanted and legitimate emails.
Improving your deliverability and maintaining a strong SenderScore requires consistent effort and adherence to best practices. First and foremost, focus on building and maintaining a healthy email list. Implement double opt-in for new subscribers to ensure they genuinely want to receive your emails. Regularly clean your list to remove inactive users, invalid addresses, and any potential spam traps. This proactive approach reduces bounce rates and minimizes the risk of hitting spam traps, both of which can severely impact your reputation.
Beyond list hygiene, ensure your email authentication records (SPF, DKIM, DMARC) are correctly configured. These technical setups verify that you are authorized to send emails from your domain, which is a fundamental trust signal for ISPs. Regularly monitor your email deliverability metrics, including open rates, click-through rates, and complaint rates. Tools like SenderScore and Google Postmaster Tools offer valuable insights into your sending performance and reputation. Addressing any dips or issues promptly is key to maintaining a strong sending profile.
Furthermore, consistent and relevant content is vital. Avoid sudden, large spikes in sending volume, as this can trigger spam filters. Instead, gradually increase your volume if you are a new sender or are adding many new subscribers. Focus on creating valuable, engaging emails that your audience wants to receive. Personalize your messages and segment your audience to deliver highly relevant content. Always provide a clear and easy way for recipients to unsubscribe, as this reduces the likelihood of spam complaints and helps maintain a positive relationship with your subscribers. By consistently applying these practices, you can improve your deliverability and build a strong, lasting sender reputation.
Interpreting SenderScore and its limitations
Poor practices
Purchased lists: Often contain invalid addresses and spam traps.
Infrequent sending: Leads to inactive subscribers and potentially higher bounces.
Missing authentication: Increases the likelihood of emails being flagged as suspicious.
Generic content: Less engagement and higher spam complaints.
Best practices
Opt-in lists: Build lists with consent, ideally using double opt-in.
Consistent sending: Maintain a regular sending schedule and volume.
Strong authentication: Implement SPF, DKIM, and DMARC for domain validation.
Personalized content: Tailor messages to recipient interests for better engagement.
A low SenderScore, or a sudden drop, can indicate underlying problems with your email program. It's essential to investigate the root causes when you notice a decline. This could involve reviewing your recipient feedback loops for increased complaints, analyzing your bounce rates, and checking if your sending IPs or domains have been added to any major blacklists (or blocklists). Addressing these issues swiftly is critical for recovery. Ignoring them can lead to prolonged deliverability problems and severely impact your email campaigns. For instance, if you see a sudden rise in spam complaints, it might be time to re-evaluate your content strategy or segmentation.
Conversely, a high SenderScore indicates that you are generally adhering to good sending practices. However, it's not the sole determinant of inbox placement. Mailbox providers use many other internal signals and complex algorithms that are not publicly disclosed. Therefore, while a high score is reassuring, it's still crucial to continuously monitor your actual inbox placement rates and adjust your strategy based on real-world delivery data. A tool like SenderScore is a valuable part of your deliverability toolkit, but it should be used in conjunction with other monitoring methods for a comprehensive view. For example, some mailbox providers might have specific internal rules that could still filter your emails, even with a perfect SenderScore.
For a deeper understanding of how these scores correlate with actual inbox placement, you can refer to research and benchmarks published by email industry organizations. For example, Mailjet has a guide on sender score and email reputation, which underscores the importance of a holistic approach to deliverability, rather than relying on a single metric. It's a continuous process of monitoring, adjusting, and optimizing your sending practices.
Final thoughts on deliverability
Email deliverability is a multi-faceted challenge that requires attention to both technical configurations and content strategy. While tools like SenderScore provide valuable insights into your IP reputation, they are just one piece of the puzzle. A holistic approach that includes diligent list management, proper authentication, engaging content, and continuous monitoring of your performance metrics is essential for ensuring your emails consistently reach the inbox.
By understanding the various factors that influence deliverability and actively working to optimize them, you can build and maintain a strong sender reputation. This will not only improve your inbox placement rates but also enhance the effectiveness of your email campaigns, ultimately leading to better engagement and return on investment. The email landscape is constantly evolving, so staying informed and proactive is key to long-term success.
Views from the trenches
Best practices
Maintain a clean and engaged email list by regularly removing inactive subscribers and invalid addresses.
Implement double opt-in for all new subscribers to ensure explicit consent and prevent spam trap hits.
Ensure your SPF, DKIM, and DMARC records are correctly configured and monitored.
Common pitfalls
Sending to old, uncleaned lists that contain invalid email addresses or spam traps.
Ignoring negative engagement signals, such as high spam complaints or unsubscribes.
Not implementing or incorrectly configuring email authentication protocols (SPF, DKIM, DMARC).
Expert tips
A high SenderScore is a good indicator, but not the only one. Always monitor actual inbox placement.
Spam traps are a major reputation killer. Proactive list hygiene is non-negotiable.
Engagement is paramount. If recipients aren't interacting, your emails will eventually be filtered.
Marketer view
A marketer from Email Geeks says they use the free sample of SenderScore and saw a high score, but are not sure about other factors that may be influencing deliverability.
2018-06-07 - Email Geeks
Expert view
An expert from Email Geeks says that a deliverability problem should not be presumed based on a SenderScore alone.
Gmail, 
Outlook, and 
Yahoo, continuously monitor various signals to determine if your emails are legitimate and desirable. These signals encompass a wide array of metrics, from the technical setup of your sending infrastructure to the way recipients interact with your messages. Each piece of data contributes to a holistic view of your sending trustworthiness, directly impacting where your emails land, or if they land at all.
Validity (formerly Return Path) that assigns a rating to outgoing mail server IP addresses. This score, ranging from 0 to 100, is designed to indicate the trustworthiness of your IP. A higher score signifies a better reputation and a higher likelihood of your emails reaching the inbox. It's often compared to a credit score, where a good score demonstrates responsible behavior over time. You can check your score through their online tool.
SenderScore and Google Postmaster Tools offer valuable insights into your sending performance and reputation. Addressing any dips or issues promptly is key to maintaining a strong sending profile.
