If you work in marketing, you’ve probably experienced it: hours spent designing a campaign, refining copy, segmenting audiences, and scheduling the perfect send time, only to find that your carefully crafted email has landed in the spam folder.
It’s frustrating, and it’s one of the biggest challenges facing email marketers today. But why does it happen, and what can we do about it?
The truth is that deliverability is about much more than great content. Spam filters are sophisticated, algorithms are constantly evolving, and what worked six months ago might not work today. Below, we’ll break down the key factors that influence whether your emails make it to the inbox or the spam folder.
1. Sender Reputation Matters
Think of sender reputation like a credit score for your domain and IP address. Internet Service Providers (ISPs) track how recipients interact with your emails: do they open them, click them, delete them without reading, or mark them as spam?
High engagement = high trust.
Low engagement or frequent spam complaints = low trust.
Over time, ISPs use this reputation score to decide where to place your emails. If your reputation slips, even high-quality campaigns can end up in spam.
2. Authentication Is Non-Negotiable
Even if your content is brilliant, without the right technical setup inboxes may not trust you. That’s where SPF, DKIM, and DMARC records come in.
SPF (Sender Policy Framework): Confirms that emails are sent from authorised servers.
DKIM (DomainKeys Identified Mail): Adds an encrypted signature to verify authenticity.
DMARC (Domain-based Message Authentication, Reporting & Conformance): Aligns SPF and DKIM to reduce spoofing and phishing.
If these are missing or misconfigured, spam filters will view your emails as suspicious and block or reroute them accordingly.
3. List Hygiene Is Critical
One of the fastest ways to damage deliverability is sending to a poor-quality list. Old, inactive, or purchased contacts often result in:
- Hard bounces (invalid addresses)
- Spam traps (emails designed to catch senders with bad practices)
- Low engagement signals
Regularly cleaning your list, removing inactive subscribers, and using confirmed opt-ins is essential. Quality beats quantity every time.
4. Content Still Plays a Role
While spam filters have moved beyond just scanning for words like “FREE!!!” or “ACT NOW!!!,” content still matters. Some common issues include:
- Image-only emails with little or no text
- Too many links, especially to different domains
- Overuse of promotional language
- Inconsistent formatting or poor accessibility
The best-performing emails are clear, balanced, and focused on value for the reader, not tricks to game the algorithm.
5. User Behaviour Shapes Deliverability
Perhaps the most overlooked factor is that the way your audience behaves with your emails directly shapes where future emails land.
If people consistently open, click, and reply to your campaigns, inboxes learn that your content is relevant. If they ignore or delete without opening, filters assume your emails don’t belong in the primary inbox.
That’s why segmentation and personalisation are so important. Sending fewer but more relevant emails can actually improve deliverability long term.
6. Deliverability Is Always Moving
What makes this area so challenging is that deliverability isn’t static. Google, Microsoft, and other mailbox providers continually update their filters. A strategy that worked brilliantly last year may underperform today.
This constant shift is why marketers need to monitor performance, test regularly, and stay informed about best practices. Tools like deliverability monitors, inbox placement tests, and reputation trackers are increasingly valuable in staying ahead.
How to Improve Your Inbox Placement
While there’s no single fix, small, consistent actions compound into significant improvements:
- Warm up new sending domains gradually
- Clean your lists regularly
- Use segmentation to increase relevance
- Check and maintain SPF, DKIM, and DMARC records
- Monitor your sender reputation
- Test before sending with deliverability tools
Final Thoughts
The spam folder isn’t the enemy, it’s a filter designed to protect users from harmful or irrelevant content. But for marketers, it can feel like a brick wall standing between your message and your audience.
By understanding the factors behind deliverability and making steady improvements, you can tip the balance back in your favour. Email marketing is still one of the most powerful tools in the marketer’s toolkit, but only if your audience actually sees your message.