Cold Emailing
CEO and co-founder
Email Engagement And Deliverability: How Opens And Clicks Impact Inbox Placement
TL;DR:
ISPs stopped trusting open rates to judge your sender reputation. With Apple MPP affecting 50%+ of opens, providers like Gmail and Outlook now prioritize replies, time-in-inbox, and authentication compliance. For agencies managing 50+ domains, this typically means you need two things working together: email copy that drives active replies and infrastructure that handles SPF, DKIM, and DMARC authentication. Skip either one, and your campaigns land in spam while your clients threaten to cancel.
Most agency founders obsess over open rates while ignoring the passive engagement signals that actually determine if their next campaign lands in the primary inbox or the spam folder. The shift happened quietly when Apple launched Mail Privacy Protection, Google tightened its spam rate requirements, and suddenly the metrics everyone relied on became unreliable.
I've seen agencies lose significant client portfolios because they optimized for the wrong signals. Their open rates looked healthy at 45%, but their reply rates hovered near zero and their authentication records had errors. The result? Deliverability can drop dramatically. This guide breaks down exactly how ISPs score your engagement, what signals actually matter, and how to build infrastructure that protects your client campaigns from the spam folder.
How ISPs measure email engagement and sender reputation
ISPs like Gmail, Microsoft Outlook, and Yahoo use sophisticated filtering algorithms to decide whether your email lands in the primary inbox, promotions tab, or spam folder. These algorithms evaluate two categories of signals:
Technical compliance: Authentication records, sending patterns, IP reputation
Behavioral engagement: How recipients interact with your messages
The decision happens in milliseconds. When your email arrives at Gmail's servers, it evaluates multiple factors including authentication checks, volume analysis, and engagement scoring based on historical data from your sending domain and IP. Understanding these filtering components helps you prioritize what to fix first when deliverability drops.
The shift from open rates to active engagement signals
Apple Mail Privacy Protection made open rates unreliable when it launched in 2021. The feature works by pre-loading message content through a proxy server, including tracking pixels, before serving them to readers. At indeterminate intervals, Apple downloads all images in your email, creating copies at a new location on the Apple Privacy Cache. This triggers your tracking pixel even if the recipient never actually opened the message.
The impact is massive. Over 50% of email opens now happen on devices with Apple's Mail Privacy Protection activated, making open rates an increasingly noisy metric for B2B cold email campaigns. As Nick Abraham explains in his video on cold email deliverability rules, the technical infrastructure requirements have changed significantly.
ISPs now weight engagement signals differently. According to FulcrumTech research, all major ISP representatives agreed that engagement is key to determining deliverability. However, ISPs do not measure clicks because it presents technological challenges and they consider click tracking a violation of user privacy.
Metric | ISP Weight | Reliability Status |
|---|---|---|
Replies | Very high | Reliable (strongest positive signal) |
Forwards | High | Reliable |
Moving from spam to inbox | High | Reliable |
Open rate | Low | Unreliable (MPP distortion) |
Deletes without opening | Negative | Reliable negative signal |
Replies are the strongest possible signal in B2B email. A reply tells the ISP that the recipient considers the email conversational and wanted. Moving an email from the junk folder to the inbox also sends a powerful trust signal, with AOL resetting previous spam classifications after just one such action.
Passive engagement and ISP filtering algorithms
Beyond replies and forwards, ISPs track passive engagement signals that most agency founders never consider:
Scroll depth: Whether the user scrolls through the entire email or just glances at the top
Read time: How long the email stays open before closing or deleting
In-email interactions: Form submissions, carousel clicks, and other embedded elements
ISPs can track interactive email engagement even without outbound clicks, particularly in webmail clients that run JavaScript to detect scrolling and read time. Gmail and Outlook collect rich, granular data on user behavior within their own code base. If recipients take time to move messages around, they indicate a level of interest that ISPs reward.
This matters for agencies because your email copy and formatting directly affect these passive signals. A wall of text that recipients delete after scanning the first line sends different signals than a well-structured email they scroll through completely. Engagement-based spam filtering creates a feedback loop where poor early engagement tanks future deliverability.
Key factors influencing your sender reputation
Your sender reputation depends on three interconnected factors: authentication compliance, sending volume patterns, and list quality. Fix one while ignoring the others, and you'll still land in spam. This is why agencies managing 50+ domains need automated systems rather than manual DNS configuration across scattered dashboards.
The role of authentication protocols (SPF, DKIM, DMARC)
Authentication protocols prove your identity to ISPs. Without them, your emails look like potential spoofing attempts, and filters treat them accordingly.
SPF (Sender Policy Framework) checks where your email came from. Your SPF record lists IP addresses authorized to send email on behalf of your domain. When you send a message, the receiving system checks for a valid SPF record. If your sending IP is on the list, you pass. If not, you fail the SPF check and could either be rejected or placed in spam.
DKIM (DomainKeys Identified Mail) cryptographically signs your emails with a key generated for your domain. Recipients' email servers grab the key from your DNS records and use it to verify your message wasn't modified during transmission.
DMARC (Domain-based Message Authentication, Reporting and Conformance) instructs receiving servers on what to do given the results after checking SPF and DKIM. A domain's DMARC policy can instruct mail servers to quarantine emails that fail authentication, reject them entirely, or deliver them anyway.
Manual setup of these records causes errors that ruin reputation. One typo in an SPF record, and your campaigns fail authentication across all 50 domains you manage for clients. Our cold email glossary covers these terms in detail for agency founders evaluating infrastructure options.
Sender volume consistency and gradual ramp-up
Sudden volume spikes trigger spam filters because they look like purchased list behavior or compromised accounts. ISPs expect legitimate senders to build volume gradually over weeks, not jump from 10 emails to 1,000 overnight.
The proper warm-up schedule according to Mailgun's domain warm-up guide:
Days 1-5: Start with 10-20 emails per day targeting your most engaged recipients
Days 6-14: Increase volume by 20% daily while monitoring bounce and complaint rates
Days 15-21: Continue gradual increases, watching for deliverability issues
Day 21+: Maintain or optimize based on performance data
Target your most engaged recipients during warm-up to ensure good open and click rates, because Gmail uses engagement as one of its main metrics for determining mailbox placement. Industry best practices recommend granting at least four weeks to email warming before launching full campaigns.
For agencies onboarding new clients, this means planning 3-4 weeks of warm-up before full campaign launch. The Inframail help center covers inbox warming after migration with specific schedules based on sending volume targets.
List hygiene and spam trap avoidance
Recycled spam traps are email addresses that must be inactive for a minimum of 12 months before inbox providers convert them into traps. When someone sends email to an abandoned address, providers return a hard bounce signaling removal. Responsible senders honor this and delete bouncing addresses. Those who ignore it and continue emailing find that abandoned addresses stop returning hard bounces and become active spam traps.
The consequences include blacklisting on Spamhaus, UCEPROTECT, and SORBS, plus damaged deliverability that takes weeks to recover.
Hard bounce thresholds matter too. A bounce rate over 2% draws ISP scrutiny, while rates above 5% can lead to penalties. The cycle compounds: poor deliverability causes more bounces, which further harms your reputation, leading to even worse deliverability. When your bounce rate crosses 2%, ISPs may start throttling your email delivery. Above 5%, you risk your domain or IP ending up on blocklists.
Strategies to improve email engagement at scale
For agencies managing 50+ domains across multiple clients, improving engagement requires three systematic approaches rather than one-off tweaks:
Optimizing copy for replies (the strongest ISP signal)
Segmenting inactive leads to protect your reputation
Testing variables without tanking your baseline sender score
The goal is maximizing reply rates while maintaining infrastructure that supports consistent sending patterns.
Optimizing subject lines and content for replies
Plain text emails consistently outperform HTML-heavy designs for cold outreach because they look like normal business communication rather than marketing blasts. Question-based copy can drive active replies by creating a natural conversation dynamic.
Effective subject lines for cold email:
Ask a direct question related to a specific pain point
Reference a trigger event (funding round, job posting, company news)
Keep under 40 characters for mobile preview
Avoid spam trigger words (free, guarantee, urgent)
The Sales Scripter team emphasizes that prospects receive many emails from salespeople, so your message must differentiate immediately. Alex Berman's guide to B2B cold email results breaks down specific copywriting frameworks that drive replies.
Your email body should end with a single, low-friction call to action. Asking for a brief call works better than requesting lengthy demos because it reduces commitment friction. The Inframail lead generation guide covers these frameworks in detail.
Running re-engagement campaigns for inactive leads
Unengaged prospects drag down your sender score. When recipients consistently ignore your emails, ISPs interpret this as low-quality sending. Segment your list by engagement and run targeted re-engagement campaigns for inactive contacts before removing them entirely.
Re-engagement framework:
Identify inactives: No opens or clicks in 60+ days
Send re-permission email: Ask directly if they want to continue receiving emails
Provide easy opt-out: Make unsubscribing simple and visible
Remove non-responders: If they don't respond to re-engagement attempts, remove from list
This protects your active campaigns by ensuring only engaged prospects receive your primary sequences. The Inframail healthy metrics guide explains what benchmarks to target.
A/B testing framework for deliverability
Test one variable at a time:
Subject line variations (test group for 48 hours)
Send time optimization (test 2-3 different windows)
Plain text vs. minimal HTML formatting
CTA placement and wording
Protect your baseline:
Wait 48-72 hours between tests for data accuracy
Track reply rates as your primary metric (given MPP unreliability)
Roll out winning variants to remaining audience after confirmation
Lead Gen Jay's video on high-volume cold email walks through the math of maintaining 50 emails per mailbox per day while scaling total volume through infrastructure, not risky shortcuts.
Deliverability health scorecard and quick wins checklist
Use this 5-point checklist to audit your current client campaigns today:
Authentication compliance
Are SPF, DKIM, and DMARC records configured correctly for all domains?
Senders typically achieve 95%+ success rates when authentication is set up correctly
Quick check: Run domains through Mail-Tester and target 9+/10 scores
Blocklist status
Is your sending IP or domain on Spamhaus, SORBS, or UCEPROTECT?
Quick check: Use MXToolbox blocklist lookup daily
Hard bounce rate
Is your bounce rate under 2%?
Quick fix: Verify all lists before upload, remove hard bounces immediately
Spam complaint rate
Are complaints below 0.3%?
Quick fix: Improve list targeting, add easy unsubscribe links
Reply rate vs. open rate
Are you tracking replies as your primary engagement metric?
Quick fix: Revise copy to end with clear questions, not statements
The Inframail spam folder guide provides detailed benchmarks for each metric.
How Inframail automates infrastructure to protect your reputation
Manual DNS configuration across 50+ domains burns 12-15 hours monthly and introduces errors that tank authentication compliance. Every typo in an SPF record, every forgotten DKIM key rotation, every misconfigured DMARC policy sends your campaigns to spam while you scramble to figure out why inbox rates dropped.
Inframail handles the technical baseline automatically. When you add a domain, SPF, DKIM, and DMARC records configure with zero manual panel work.
The economics work for agencies at scale. Our dedicated IP comparison explains why Google Workspace shared pools create reputation risk from other senders' behavior.
Infrastructure | 50 Inboxes | 100 Inboxes | 200 Inboxes |
|---|---|---|---|
Google Workspace ($7-8.40/seat) | $350-420/mo | $700-840/mo | $1,400-1,680/mo |
Inframail Unlimited Plan | $129/mo | $129/mo | $129/mo |
Annual Savings | $2,652-3,492 | $6,852-8,532 | $15,252-18,612 |
Inframail includes dedicated US IPs (1 on Unlimited Plan, 3 on Agency Pack), isolating your sender reputation from shared pool problems. Kidous covers the dedicated vs shared IP differences in detail.
The flat-rate pricing protects your margins as client count scales. Another reviewer confirmed this value:
"I personally have over 1,000 email accounts with Inframail for one flat price. Adding all those records would have probably taken dozens of hours. Instead all records were added within 10 minutes." - User review of Inframail (38 5-star reviews on Trustpilot)
Sign up to Inframail and get started today.
Frequently asked questions
What spam rate threshold triggers ISP throttling?
Google requires bulk senders to keep spam rates below 0.3%. Exceeding this threshold consistently leads to throttling and potential blocklisting.
How long should domain warm-up take before full campaign volume?
Plan for at least four weeks of gradual warm-up. Start with 10-20 emails on day one and increase volume by 20% daily while targeting your most engaged recipients.
What hard bounce rate signals deliverability problems?
A bounce rate over 2% draws ISP scrutiny, while rates above 5% typically lead to penalties including potential blocklisting on services like Spamhaus.
How many dedicated IPs does Inframail provide?
The Unlimited Plan ($129/month) includes 1 dedicated US-based IP. The Agency Pack ($327/month) includes 3 dedicated IPs for agencies managing higher volumes across multiple clients.
Does Apple Mail Privacy Protection affect cold email deliverability directly?
Apple MPP makes open rates unreliable for measurement but does not directly harm deliverability. ISPs now weight replies and other engagement signals more heavily as a result.
Key terminology
Sender Score: A numeric rating (0-100) that represents your sending reputation based on email volume, complaint rates, spam trap hits, and engagement metrics. Scores below 70 typically result in filtering or rejection by major ISPs.
Recycled Spam Trap: An email address that was once used by a real person, became abandoned for 12+ months, and was converted by the inbox provider into a trap to catch senders with poor list hygiene.
Dedicated IP: A sending IP address assigned exclusively to your organization, where your behavior alone determines reputation. This contrasts with shared IP pools where one bad actor can damage deliverability for all users.
Domain Warm-up: The process of gradually increasing email volume from a new domain over 3-4 weeks to establish positive sender reputation with ISPs before launching full campaigns.
ISP Throttling: The practice of major email providers (Gmail, Outlook, Yahoo) intentionally slowing or limiting delivery of emails from senders with poor reputation metrics, resulting in delayed delivery or inbox placement penalties.
