The success of any cold email campaign relies on the quality of its deliverability. When deliverability drops, issues start to cascade. If you don’t find the root cause, domains get burned, and you begin the entire email infrastructure set-up process all over again.

In this guide, we’ll show you how to achieve and maintain pristine deliverability, scale campaigns effectively, and resolve issues as they arise. Let’s get started. 

What Causes Poor Deliverability?

Email deliverability issues stem from various factors. If you’re currently running or about to launch a campaign, be aware of the following red flags that cause email deliverability to tank:

Shared IP Pool Burned

One of the most common (and also the most overlooked) reasons why deliverability drops is your shared IP pool. If another sender on that pool starts sending low-quality or spammy campaigns, spam filters do not try to isolate their traffic from yours.

Here’s how to know you have a burned shared IP pool:

• Your setup appears to be correct on paper: SPF, DKIM, and DMARC are all valid, the sending domain is clean, and you are not listed on any obvious domain-level blocklists.
• Different providers behave differently: Gmail might still place your campaigns in the inbox, while Outlook, Yahoo, or certain corporate domains may start sending your campaigns straight to spam or quarantine.
• Performance drops: Open rates decline across lists and segments simultaneously, even though the audiences are distinct and your targeting has not changed.

What to do if you suspect the shared pool is burned:

• Confirm it’s not you: Send the same email from your native inbox and from your platform. If the native inboxes and the platform version hit spam, it’s infrastructure-related.
• Check IP reputation: Look up the sending IP address your platform uses and see if it’s on blocklists or associated with spammy traffic.
• Push your provider: Ask if you’re on a shared pool and request a move to a cleaner pool or dedicated IP.

New or “Too Young” Domains

Email providers treat new domains as high risk because they haven’t yet built up their sender reputation. Many admins auto-block or heavily filter domains under 30 to 60 days old, and brand-new O365 tenants are often throttled.

On your end, the message shows as “delivered,” yet the recipient never receives the email in their inbox or spam folder.

To avoid that, you can:

• Age domains before serious sending: Register domains early, use them for low-volume, real conversations, and build a history of standard human replies.
• Warm up slowly: For the first 60–90 days, keep sends low, keep copy personal, and focus on contacts likely to respond.
• Watch new O365 tenants closely: If you are on a new Microsoft tenant, verify with Microsoft that there are no hidden “trial” or anti-abuse limits suppressing your outbound.

Weak Engagement and Tiny Lists

Email providers judge you heavily on engagement signals. On a small list, a couple of spam complaints or poor open rates look like red flags. Even if your setup is perfect, low opens and a few “this is spam” clicks damage your domain reputation.

This hits hardest on tiny lists. Suppose you have only 100 to 300 contacts, and one or two people marking you as spam suddenly accounts for 1%–2% of your entire list. Filters see that percentage, not your intentions.

Here are a few fixes you can try:

• Keep engagement segments tight: send core campaigns to people who have opened or clicked within the last 30 to 90 days, instead of blasting your entire list.
• Run a simple sunset policy: try a short re-engagement sequence, then suppress subscribers who never open or click.
• Make it easy to unsubscribe: a clear, visible opt-out link is better than forcing people to use the spam button.
• Prioritize replies and micro-conversions: ask simple questions or invite direct replies so providers see real two-way interaction.

Email Deliverability Checklist: Before, During, & After Campaigns

Most deliverability problems happen because something was overlooked before sending, ignored during campaigns, or left unaddressed afterward.

Instead of treating deliverability as a one-time setup step, use this checklist before, during, and after every campaign.

Before Campaigns: Fix the Foundations

Before sending a single cold email, ensure your technical setup and lists are safe for inbox providers, secure email gateways, and managed service providers.

Infrastructure and authentication

• Confirm SPF, DKIM, and DMARC are correctly set up and aligned with your domain.
• Ensure your domain isn’t brand new; if it is, keep the early sending volume very low.
• Check if you’re on a shared IP pool and if that IP has any blacklist or reputation issues.
• Separate “core brand” email from cold outbound using different domains or subdomains.
• Set up custom tracking domains instead of using generic platform tracking links.

Lists, segments, and copy

• Use verified, cleaned leads instead of scraped or unvalidated lists.
• Segment by engagement (e.g., opened/clicked in the last 30 to 90 days).
• Define a basic sunset policy for non-openers before you start.
• Keep copy personalized and natural so it reads like a 1:1 email, not a mass blast.
• Limit links and images at the start and avoid spammy, hype-heavy subject lines.

During Campaigns: Monitor Signals & Protect Reputation

Once campaigns are live, treat deliverability like a metric you actively manage. Here are the factors you should check:

Monitor health in real time

• Track opens, replies, bounces, and spam by provider (Gmail, Outlook, Yahoo).
• Run inbox placement tests to see where you land: primary, promotions, or spam.
• Monitor IP and domain reputation, and be vigilant for any new blacklist entries.

Adjust quickly when you see problems

• Pausing or slowing down sends to providers can cause open rates to drop or increase spam folder placement suddenly.
• Cut volume temporarily if engagement drops, then rebuild with smaller, more engaged segments.
• Turn off or reduce tracking (opens/clicks) and simplify templates if tests start going to spam.
• Prioritize campaigns that earn replies and positive engagement to send stronger “this is wanted” signals.

After Campaigns: Clean Up & Rebuild Stronger

After each sending cycle, clean up your data, verify all new contacts, and use what you learned to improve the next round.

Clean lists and suppress risk

• Remove or suppress hard bounces and contacts that are repeatedly inactive.
• Run re-engagement for semi-cold leads, then sunset those who still never open or click.
• Review spam complaints and unsubscribe patterns to identify which angles or audiences are causing issues.

Review strategy and infrastructure

• Check performance by domain, segment, and campaign to understand where reputation is strongest.
• Determine if you need to move off a risky shared IP pool, add more sending domains, or adjust the warm-up period.
• Update your sending limits, ramp schedules, and engagement rules based on what actually worked.

Key Takeaways

Deliverability is arguably the most crucial metric in any email campaign, and it’s something you need to track consistently. Here’s a quick overview of our email deliverability checklist for before, during, and after each campaign:

• The primary causes of poor deliverability include being in a shared IP pool, launching campaigns with new domains without a warm-up, spammy email copy, and a small list. 
• Before starting campaigns, ensure domains are authenticated and warmed up for at least two weeks. When warming up, start with a low volume and gradually increase it.
• Monitor deliverability during your campaign to identify any potential issues early. You can examine metrics such as open, spam, and bounce rates. 
• After campaigns, review your strategy and cold email infrastructure. That means cleaning lead lists, checking domain performance, and key email marketing metrics.

If you want to scale email marketing campaigns and cold outreach, you need a tool that adjusts campaigns based on deliverability in real time. Instantly provides just that and much more. Try it for free today.