Email Deliverability Monitoring That Flags Problems Before Google Does

Do you know that a delivered email doesn't automatically land in the inbox?

Real-time email deliverability monitoring is one of the strongest metrics that will lead to conversions. The way it works is simple: When you monitor email deliverability metrics, you get a clear picture of how your campaign is performing and any potential deliverability holdups. Now, you may have your SPF, DKIM, and DMARC set up, and that is great, but it is not enough. In here, we'll break down what email deliverability monitoring actually means and how tools like Aerosend help the best cold email agencies in the world monitor inboxes while maintaining their entire email infrastructure.

What Is Email Deliverability Monitoring?

Before we get into why you should monitor your email deliverability and what metrics to look for, let's dissect it a bit more. Email deliverability monitoring is the process of tracking where your emails are potentially landing by identifying patterns. These patterns help you catch advanced issues, including domain fatigue and poor sender reputation, before they affect performance.

Preface: No one knows where your email landed in the Inbox or SPAM, except the person who received it. However, what you CAN do is follow patterns and identify them to check where the problem lies.

To monitor email deliverability, you're watching:

• Inbox placement
• Bounce rate/Bounce Type
• Engagement signals

However, monitoring these metrics all by yourself can not only be challenging but also exhausting. Luckily, Aerosend has developed a tool that automatically monitors email deliverability in real-time for our clients.

Our one-of-a-kind system runs warmup signals, domain health checks, blacklist scans, and even reroutes traffic when infrastructure degrades. Why? So, your team doesn't have to manually spot every dip. It is your one-stop shop for all your cold emailing needs.

Email Deliverability Metrics to Monitor

The thing about email deliverability is that it can quietly drop, no matter if you're running a campaign for 1 client or 20 of them. This is precisely why the email deliverability metrics are important to monitor; it’s to check if you’re getting the most out of your email campaigns. Take a look at what experienced senders track regularly:

1. Warmup Score

While this is not the most reliable tool to monitor your email deliverability, you can still gauge how your domain is doing. Providers own the accounts they use to run warm-ups, so they can tell you if your email lands in the inbox or not. This is not a recommended way to track deliverability, as most tools provide a warm-up score based on how your emails perform in their internal pool.

2. Reply Rates by Domain

When your emails get replies, it is a sign that your message ended up where it was intended to. However, unlike most people, you need to track your reply rates per domain instead of campaign to actually get an insight into how your domain is doing.

3. Bounce Rate by Domain

If you are rotating domains, it is a good practice to monitor the bounce rate per sending domain, rather than just across campaigns. In other words, your campaign not performing well may not be because of a badly designed campaign, but because of domain issues. This is because not all domains are equal. You need to focus on two types:

• Mailbox Not Found: Usually means poor list quality.

• SPAM Reject Bounces: These indicate that the receiving server is blocking your emails.

4. Open Rate by Domain

At one point, open rates were a core metric, but today they’re increasingly unreliable, especially with Apple Mail Privacy Protection and other provider-side changes. While you can view open rates, we don’t recommend using them as your primary signal. They can still be directionally useful in certain cases.

5. Inbox Placement Test

An inbox placement test gives you an insight into how different email providers manage different messages. The test shows you how many of your messages land in the recipients’ inboxes across various providers.

6. Blacklist Appearance Alerts

If there is one list you don't want to be on, it's the DNSBL (Domain Name System-based Blackhole List) because it can instantly cripple your deliverability, even if your copy is clean.  You don’t need to check manually. Just make sure you're running automated checks for it.

How to Monitor Email Deliverability the Right Way

To make sure that your email campaign is a success, email deliverability monitoring is essential and not optional. Here's exactly how you do it.

1. Monitor Warmup Reputation (Continuously)

Domains need ongoing warmup to maintain trust with inbox providers. A good reputation may not guarantee inbox, but a bad one makes sure that you never end up in one. If your warmup score ever goes below 90, it is a clear sign that you need to stop sending emails and let the domain recover for at least two weeks. If the score doesn’t recover, swap out the domain (assuming you’re rotating domains). So, use the score as a signal to slow down, not as a single source of the truth.

2. Biweekly Inbox Placement Tests

Your emails might be "delivered," but are they landing where you want them to? Inbox placement tools use seed accounts to test if your email ended up in the right place. The catch here is that seed accounts do not behave like real-world accounts. Also, unlike real-world accounts, they don’t have any security defence systems. So, a “good” inbox rate doesn’t mean much, but bad results (like spam) are a definite red flag.

• Aerosend runs bi-weekly inbox placement tests. So you see exactly how many of your emails land, not just if they were sent and provide a solution for the same.
• 1st Week SPAM, stop sending
• 2nd Week SPAM, change domain

3. Track Engagement by Domain (Weekly)

Let’s break this down into three types:

A. Track Reply Rates by Domain

Not all domains perform equally. That’s why reply rate should always be tracked per domain. If a domain consistently underperforms, it’s likely already compromised.

• Standard practice: remove the bottom 10–25% monthly to prevent deliverability drag.
• Don’t judge based on low volume. It is recommended to wait until you’ve sent at least 500 emails per domain before deciding.
• Aerosend tracks reply rates by domain and flags underperformers.</aside>

B. Track Bounce Rates by Domain

You need to track your bounce rates by domain instead of per campaign to monitor domain health.

• Aerosend monitors bounce rates per domain weekly.
• If bounce rates spike on a specific domain, we remove it before reputation damage spreads.

C. Track Bounce Type

Again, not all bounces are created equal. Focus on two high-signal types:

• Mailbox Not Found
• Usually caused by bad lists or outdated data. Clean and verify before sending.
• SPAM Reject Bounces
• More serious. These indicate your emails are being blocked due to reputation or spam signals. If you’re seeing this, stop sending from that domain immediately and consider rotating it out.

4. Blacklist

You definitely don’t want to be on any Blacklists. While there’s many of these lists available, you only need to be on the lookout for the following ones Blacklists:

• Spamhaus
• Spamcop
• Barracuda
• SEMFRESH

If you appear on any of the above lists, it is advised to stop your campaign and investigate the root cause.

Aerosend flags early signs of domain fatigue and deliverability drops as part of its built-in infrastructure. You get real-time alerts before your campaigns are impacted. Rather than stitching together separate tools for monitoring, warmup, inbox placement, and rotation, Aerosend seamlessly monitors email deliverability automatically for our client inboxes.

Why Most Tools Don't Monitor Deliverability Properly

Now that we know what metrics to follow, let's check why most tools, despite promising results, still fail in doing so.

1. They Rely on Misleading Metrics

Most tools prominently feature “delivery rate” as a success metric, but this number is misleading. It simply shows whether the receiving server accepted the email. It says nothing about where the email actually landed or if it was filtered out entirely.

In other words, a 98% delivery rate surely sounds healthy, but if half those emails are in spam and your campaigns are underperforming, your domain reputation may already be in decline.

2. They Use Low-Quality Warmup Pools

Traditional warm-up tools use shared pools of seed and cold email accounts, but they come with their own set of problems. In reality, more than 60% of these inboxes are already known to be part of cold email networks. That means inbox providers don’t treat those emails like real user interactions.

Your tool might show you’re “inboxing” during warm-up, but that success doesn’t translate to real-world campaigns. These warm-up results create a false sense of security. You go live with your campaign, assuming everything is fine, until Gmail or Outlook starts silently filtering your emails.

3. Inflating Important Metrics

Some tools will give you an inaccurate picture of what is really going on. In fact, they rarely provide visibility into metrics that are important if you’re running domain rotation or managing high-volume sends. You may receive a stat about your reply rate that seems positive, but here is what is actually going on.

For example:

You send 10 emails to the same lead. They reply once.

Your real reply rate is 1 reply / 10 emails = 10%.

Most tools will report it as 100% (1 reply per contact).

To explain futher,

You could send 20 emails to 2 leads (10 per lead) and get 2 replies (1 per lead), and the system will show a 100% reply rate because both of your leads replied. However, this isn’t an accurate picture of how much effort it took to get that one reply.

What is even more problematic is that they may further omit to include details like reply rates by domain and bounce rates by domain. These metrics are especially important when you’re trying to monitor your domain health.

This is harmful when you're evaluating which domains or copy variants to pause or scale, because you're optimising based on inaccurate data.

See: Aerosend utilises actual reply rates for email deliverability monitoring, so you know what’s working and where engagement is slipping.

4. Pre-Send Checks Lack Post-Send Insight

Some tools check pre-send factors such as authentication and content scanning, and while they are helpful, they don't track post-send reputation shifts.

You might pass SPF/DKIM tests, but that won't protect against domain fatigue or IP reputation decay since these issues surface only after sustained sending.

We understand that all of this can feel overwhelming, especially with cold emailing having so much else to not only look out for but also monitor. That is where Aerosend comes in.

Aerosend's Built-In Email Monitoring System

Most email deliverability monitoring tools offer surface-level stats, but Aerosend goes far beyond that. We build and manage the full cold email inbox infrastructure for our clients, with deliverability monitoring built into the core.

We continuously track the health of your domains and check inbox placement. We do it all before, during, and after your campaigns go live. That means issues get flagged before they affect performance.

Here’s what makes Aerosend’s deliverability monitoring different:

Domain Reputation Tracking

Know exactly how each sending domain is performing. Aerosend alerts you the moment your reputation dips, so you can pause or rotate before inbox placement is affected.

Auto Alerts & Smart Rerouting

When IP or domain performance drops, Aerosend automatically reroutes traffic through healthier channels. No manual monitoring or guesswork required.