What No One Tells You About Cold Emails Going to Spam

In the cold emailing world, a lot of the emails probably deserve to go to spam…but the one that you put a lot of effort into crafting and getting a good lead? They sure don’t. So, if you’re wondering why your emails are going to spam, you’re not alone.

This is especially true when cold emails going to spam becomes a pattern you can’t afford to ignore. The reality is, emails going to spam don’t just affect deliverability; they destroy trust. It could be your first touch with a cold lead or a follow-up with a potential customer; ending up in the spam folder means the message never lands, and if cold emails go to spam, the entire sequence fails before it even begins. However, there are ways you can prevent this. You see, spam filtering is about trust, and cold emails start with none. Since you haven’t interacted with the recipient before, email providers assume your message could be suspicious.

In this article, you’ll learn why cold email often gets flagged, how domain reputation impacts your inboxing, and most importantly, how to prevent emails from going to spam using a step-by-step system that the experts at Aersond use!

Why Cold Email Often Goes to Spam

Platforms like Google and Outlook don’t make it easy for senders; they evaluate them using a mix of signals, many of which are invisible to the average sender. You need to understand why cold emails are going to spam by understanding what inbox providers actually care about.

One of the biggest factors? Your domain reputation. Think of it as a credit score for your domain, but instead of banks deciding whether to trust you with a loan, it’s email providers deciding whether to trust you with their users’ inboxes. That score makes all the difference between inboxing and cold emails going to spam.

Many factors could influence this score. If your domain is relatively new, contains spammy content, lacks SPF, DKIM, or DMARC, has low engagement rates, or faces other issues, your score will likely suffer. The result is you getting buried in the spam folder.

In short, cold emails are flagged because of poor reputation, weak setup, or bad engagement; sometimes all three. In fact, even high-quality senders can find their emails going to spam if reputation metrics dip. Fixing that starts with identifying these issues early and making infrastructure changes before sending another email.

Why Your Emails Go to Spam (and How to Prevent It)

Most people blame their copy or even their entire campaigns when it doesn’t give the expected result. The truth is that your emails might still go to spam even when your copy is perfect. Why? Because inbox providers care more about sender behaviour than your message. Take a look at some of the major reasons why your cold emails are going to spam, along with how to fix each one.

1. No Warm-Up or New Domain

When you join a gym, you don’t walk in on the first day and do 300 deadlifts. You start slow, learn proper form, build consistency, and earn the trust of your muscles. Domains also work on the same principle. Send too much too soon, and you'll pull something (like your deliverability). Start small, earn inbox trust, and scale smart.

Fix: Follow a proper warm-up process over 2–4 weeks. It is essential to have a trusted warm-up provider to warm up your inboxes. Warmup pools with a very high number of cold email accounts or accounts that are blacklisted might affect deliverability. Aerosend uses Warmupinbox.com to warm up your inboxes (Complimentary for all Aerosend users)

2. Missing SPF, DKIM & DMARC

Email authentication can be tricky, but it plays a significant role in your sender reputation. To put it simply, if you don’t authenticate your emails, providers cannot verify who you are. The end result of this would be being rejected outright, which sends your email to spam. Proper DNS setup is important if you want to prevent emails from going to spam.

• SPF defines which IPs are allowed to send on behalf of your domain.
• DKIM adds a cryptographic signature to prove the message hasn’t been altered.
• DMARC tells mailbox providers how to handle unauthenticated messages.

Fix: Set up all three properly. You can do it yourself or let Aerosend handle it for you. We handle it automatically during onboarding for our clients, configuring DNS records so your emails pass those authentication checks before you hit send.

3. High Bounce Rates

If you’re already running campaigns and have a high bounce rate, it tells your inbox provider that you’re sending to bad or unverified addresses. It’s also one of the clearest signs that your list quality is poor. Even with a seemingly clean list, just a few bad leads can result in cold emails going to spam. That’s why there are two bounce types to watch:

• Mailbox Not Found: This typically means you’re sending to invalid or outdated contacts. You need to verify your lead list regularly before every campaign.
• SPAM Reject: This happens when the receiving server outright blocks your email because your domain is considered spammy (E.g. - Sudden spike in sending volume).

Fix: Use a tool like Aerosend to track bounce types in real-time. If you see SPAM rejects, immediately stop sending from that domain. Replace it with a fresh one. Also, recheck your content and lead source to rule out spam triggers and low-quality data.

4. Spammy Content or Subject Lines

We all love to market our product in our emails. The whole point of sending one is to get a conversion. However, back in the early days of spam filtering, your content used to be the main target. They still look for trigger words (Limited edition, buy now, 15% off, free, winner, and more). Additionally, an overuse of emojis or bad HTML, blacklisted links, or poor formatting, could be just some of the reasons why you aren’t ending up in the primary inbox. Even if you pass all technical checks, spammy copy can still result in emails going to spam, especially with subject lines that trigger filters.

Fix: Keep your subject lines short and to the point, no all caps in the content, no exclamation points, and no media or links (at least for your first email). Avoid banned words, keep your copy short, and ALWAYS test your emails before launch. Aerosend helps its clients run a bi-weekly inbox placement test to check if their emails are landing where they want them to.

5. Poor Engagement Signals

One of the most overlooked reasons for cold emails going to spam is low engagement. Mailbox providers measure your engagement rates. What this means is that they check the emails you send to see the reply rates, open rates, and the receiver's actions, such as marking you as spam. This ultimately ends up becoming a vicious cycle because if you have poor engagement rates, your chances of landing where you want to are already low, if not completely gone. In other words, low reply = low deliverability, and this keeps going on.

Fix: Track reply rates by domain, not just campaign-level stats. Remove the lowest 10–25% of performing domains every month. Wait until at least 500 emails have been sent before making a decision. Aerosend tracks this weekly and automates domain rotation for you.

6. Cold Email Volume Spikes

What if your friend sends you one message every day, and then one day you wake up with 1000 messages from them? Suspicious, right? The same applies to your cold emailing campaigns. Sudden volume increases are a surefire reason for cold emails going to spam, especially on new domains. Inbox providers expect:

• Gradual send growth
• Consistent daily patterns
• Natural user behaviour

Random volume spikes are how spam bots behave, and you don’t want to end up in that category, especially by mass sending from a single domain.

Fix: Start slow and ramp up over time. Use multiple domains and multiple inboxes per domain. A good starting setup would look something like this: Ten domains, each with three inboxes, start with 25 emails per inbox per day. Aerosend helps with warm-up (warmupinbox.com, complimentary for our clients), rotation, and pacing, which are fully automated. Rest assured, your outreach will always be within safe limits.

7. Blacklists

The worst thing that can happen to a cold emailer is landing on on a DNSBL (Domain Name System Blacklist). Shared across inbox providers, these lists severely impact your ability to reach any inboxes. Blacklisted domains almost guarantee cold emails going to spam until reputation is repaired.

Fix: The good thing here is you don’t have to do much, just run automated checks for lists like Spamhaus, Spamco, Barracuda, and SEMFRESH. Aerosend runs blacklist scans and alerts you immediately if your IP or domain gets flagged. From there, we stop sending emails from the affected source and reroute them automatically, preventing performance issues.

How to Prevent Emails from Going to Spam: A Simple Breakdown

Advanced Infrastructure Tips – What Others Miss

You’ve fixed all you could, but are your cold emails going to spam, still? Once you have your basics out of the way, you can start focusing on the advanced tips on how to maintain your email infrastructure so your emails stop going to spam. If you’re not accounting for how different providers filter email, especially Outlook, you’re losing out on valuable inbox reach. Unlike Gmail, which focuses more on engagement and content, Outlook has strict anti-spam defences that rely heavily on IP history, domain reputation, sender volume, and user-level security filters.

The Sine Wave Pattern (The On/Off for your Domains)

To maintain high inbox placement and avoid domain fatigue, Aerosend developed a sine wave pattern so that our clients could get the most out of their domains without burning them. This sending involves strategically rotating sending domains, pacing campaigns, and scaling only when performance signals are strong. This rotation model helps avoid fatigue and stops cold emails going to spam during scale-up. Here's how the system works step-by-step:

Step 1: Calculate and Divide Total Domains

Let’s say you have 20 domains and want to send 1,500 emails per day, limiting each domain to 75 emails. Now, we divide our domains into two sets, each of which has 10 domains. So, we have two sets.

• Total: 20 Domains = 1,500 Emails/Day
• Split Into Sets:
  • Set #1: 10 Domains = 750 Emails/Day
  • Set #2: 10 Domains = 750 Emails/Day

This kind of separation allows controlled delivery and helps reduce the risk of reputation dips.

Step 2: Add Tags

Tagging leads based on variables like persona, industry, or stage of funnel helps you create tighter, more targeted campaigns. It also allows you to test different messaging approaches more effectively.

Step 3: Divide Campaigns & Lead List

Once you have your domains and tags figured out, it’s time to split your lead list according to the length of your sequence:

• Over 2 weeks:
  • 750 emails/day × 10 days = 7,500 emails
• For a 1-step sequence:
  • You’ll need 7,500 leads
• For a 2-step sequence:
  • 7,500 ÷ 2 = 3,750 leads
• For a 3-step sequence:
  • 7,500 ÷ 3 = 2,500 leads

Now that you have your sets divided and warmed up, you will pick set one to launch your campaign while set 2 is still in warm-up. We recommend using each set for two weeks before switching out.

Step 4: Launch New Campaigns Every 2 Weeks

You need to reduce the risk of domain fatigue and keep engagement fresh. So, create entirely new campaigns every 2 weeks. This keeps the sending pattern natural and signals good behaviour to email service providers (ESPs).

Step 5: Don’t Use All Inboxes at Once

Coming back to your inboxes/domains, hold back some of your inboxes. Only use 50% of your available inboxes during the initial testing phase to find message-market fit.

Once you see consistent positive replies or conversions, scale up by adding more inboxes. (Two sets with 10 domains each, use only one set at a time)

Step 6: Run Multiple Campaign Types

Don’t rely on just one approach. Try running multiple campaign variants in parallel to test what drives engagement.

• Hard CTA Campaigns
• Loom Video Campaigns
• Lead Magnet Campaigns

Different formats can help appeal to different personas and uncover which message style resonates best.

Step 7: Keep Backup Domains Ready

Always keep 20%–50% of your domains on standby as backups. These domains are your safety net, and they need to be ready to be activated in case you need to pause fatigued domains or respond to a drop in inbox placement.

Bonus: Unconfirmed Tactics We’re Still Testing

These aren’t fully validated yet, but early signs are promising:

• Campaign Start Time Variation: Varying when your emails start sending may reduce pattern detection and improve deliverability.

• Sending to the Best Leads First: Prioritise top-quality leads early in the campaign to increase your chances of early replies, which can boost domain reputation.