Sender score is a numerical reputation rating assigned to your email sending IP address, scored on a scale of 0 to 100. Originally developed by Return Path (now part of Validity), it functions as a credit score for email senders - the higher your score, the more likely email providers are to trust and deliver your messages.
How Sender Score Is Calculated
Sender score evaluates your sending IP based on a rolling 30-day window of email behaviour. The key factors include:
Complaint rates. The percentage of recipients who mark your emails as spam. Even small complaint rates have a significant impact. Rates above 0.1% begin to lower your score noticeably.
Bounce rates. The percentage of emails that bounce - particularly hard bounces from invalid addresses. High bounce rates signal poor list quality and are one of the fastest ways to lower your score.
Spam trap hits. Sending to addresses that are known spam traps - addresses maintained by email providers and anti-spam organisations specifically to identify senders using unverified lists. Even a small number of spam trap hits can severely damage your score.
Blacklist presence. Whether your IP appears on public blacklists maintained by organisations like Spamhaus, Barracuda, or SURBL. Blacklist listings are both a cause and consequence of poor sender score.
Sending volume and consistency. Sudden spikes in volume or erratic sending patterns lower your score. Consistent, predictable sending volume is a positive signal.
Infrastructure quality. Proper authentication (SPF, DKIM, DMARC), valid reverse DNS records, and correctly configured mail servers all contribute to a higher score.
What the Numbers Mean
Sender score ranges and their practical implications:
- 80-100: Excellent. Your emails will be delivered to the inbox at most providers. You have earned trust through consistent, clean sending behaviour.
- 70-79: Good but watch closely. Most emails will be delivered, but some providers may filter more aggressively. Minor issues in your data or sending practices are starting to show.
- 50-69: Problematic. A significant portion of your emails will land in spam or be throttled. You need to identify and fix the underlying issues - typically data quality or complaint rates.
- Below 50: Poor. Most email providers will filter, throttle, or block your messages. Recovery requires a sustained effort to clean your data, reduce complaints, and rebuild trust over weeks or months.
Sender Score vs Domain Reputation
Sender score measures IP reputation. Domain reputation measures the reputation of your sending domain. Both matter, but they work differently.
If you use a shared IP (common with email service providers like HubSpot, Mailchimp, or ActiveCampaign), your sender score is influenced by all senders on that IP - not just you. This means another sender's poor behaviour can affect your deliverability.
If you use a dedicated IP, your sender score reflects only your sending behaviour. This gives you more control but also more responsibility - there is no one else's good behaviour to offset your mistakes.
Domain reputation follows your domain regardless of which IP you send from. Even if you switch IPs or email providers, your domain reputation persists. For B2B outbound, domain reputation has become the more important factor as email providers have shifted their filtering algorithms to weight domain signals more heavily than IP signals.
How to Check Your Sender Score
Validity Sender Score (senderscore.org). The original and most widely referenced tool. Enter your sending IP address to see your current score and historical trends. Free to use for basic checks.
Google Postmaster Tools. Shows your reputation specifically with Gmail. Does not give a numerical score but provides a High/Medium/Low/Bad rating that correlates with deliverability.
Microsoft SNDS. Shows your reputation with Outlook and Microsoft email services. Particularly important for UK B2B where Outlook dominates corporate email.
To find your sending IP, check your email platform's settings or send a test email and examine the message headers. The sending IP is listed in the "Received" headers.
The Connection to Data Quality
Sender score is ultimately a reflection of your data practices. The two biggest factors that lower it - bounce rates and spam complaints - are both direct consequences of data quality.
If your contact data is accurate and current, bounces stay low. If you are reaching the right people with relevant messages, complaints stay low. If your data is decayed and your targeting is poor, both metrics deteriorate and your sender score drops accordingly.
This is why improving sender score is not primarily a technical exercise. You can optimise your email infrastructure, configure authentication perfectly, and follow every best practice for sending patterns - but if you are sending to a list where 8% of addresses are invalid due to data decay, your sender score will suffer regardless.
How ClientWise Applies This
We treat sender score as a downstream metric of data quality. By ensuring every email address is SMTP-verified before it reaches your CRM and maintaining ongoing validation cycles through our data ops retainer, we address the root causes of sender score degradation rather than the symptoms.
Our CRM health check includes an assessment of your current bounce and complaint rates and their likely impact on your sender score. For clients experiencing deliverability issues, we often find that data quality improvements produce measurable sender score recovery within four to six weeks.