How ISPs judge your emails: reputation, engagement, spam traps, complaints
Joey Lee
September 8, 2025
When we talk about inbox providers, we often call them ISPs, short for internet service providers. It is a bit of an old term, but in practice it refers to the companies that control inboxes like Gmail, Yahoo, Microsoft, Apple, and others. These providers decide whether your message goes to the inbox, the spam folder, or gets blocked entirely.
The most important factor they look at is your reputation. Think of it like a credit score for your email program. Every time you send, ISPs track how your audience responds and adjust your score. A strong reputation leads to inbox placement. A weak reputation means more of your emails end up in spam.
This is usually at the core of deliverability problems. If your emails are going to spam, most of the time it is because your reputation with the ISP is poor.
Why your reputation might be poor
Low engagement: people ignore or delete your emails without opening.
High complaints: recipients click “report spam.”
Bounces: you are sending to invalid or inactive addresses.
Spam traps: you hit an ISP’s trap address, which signals bad list practices.
Volume spikes: sudden surges in sending can look suspicious.
How to know if your reputation is poor
Google Postmaster Tools will tell you directly with a sender reputation rating.
Low open rates compared to your historical baseline.
Low click rates, showing subscribers are disengaging.
Employees or customers mention your emails are landing in spam.
Bounce rates that are higher than normal.
Sudden drops in inbox placement when you seed test across different ISPs.
Blacklist checks show your domain or IP has been flagged.
How to maintain a strong reputation
Send only to people who expect and want your emails. Avoid purchased or scraped lists.
Keep your list fresh by removing inactive or bouncing addresses.
Monitor engagement and focus on sending content people consistently open and click.
Ramp up volume gradually when adding new subscribers or launching new campaigns.
Use authentication (SPF, DKIM, DMARC) so ISPs know your messages are legitimate.
How to improve a poor reputation
Pause or slow down sending until you identify the issue.
Segment your list to send only to your most engaged subscribers. This creates positive engagement signals to rebuild trust.
Remove unengaged contacts, hard bounces, and role accounts (like info@ or sales@) from your list.
Send fewer but higher quality campaigns until your reputation recovers.
Use monitoring tools like Google Postmaster Tools or Microsoft SNDS to track improvements.
Reputation is not permanent. Just like with a credit score, you can recover from past mistakes by showing consistent positive behavior over time. The key is to treat every send as a signal and make sure those signals are helping, not hurting.
In the next article (article 4), we will look at list hygiene and data quality, and why maintaining a clean list is one of the most powerful ways to improve reputation.
Intro to email deliverability
Authentication 101: SPF, DKIM, DMARC explained simply
How ISPs judge your emails: reputation, engagement, spam traps, complaints
List hygiene and data quality: why clean lists matter more than big lists
Content and design factors: subject lines, html, images, links, spam triggers
Infrastructure and sending practices: shared vs dedicated IP, warming, throttling
Monitoring and troubleshooting: how to use seed tests, blocklist checks, analytics
Future of deliverability: AI filters, gmail and yahoo changes, privacy trends