Layer 5 · Inbox Placement
Why are my emails going to spam?
The honest answer, traced through the stack. Spam folder placement is almost never about subject lines — it is almost always a reputation or authentication problem two or three layers below what you see.
The three questions every filter asks
When your message arrives at Gmail, Outlook, or Yahoo, the receiving mail system asks three questions in roughly this order:
- Is this sender who they say they are? — SPF, DKIM, and DMARC answer this. If authentication fails or is missing, the message is suspect before anyone reads it.
- What is the reputation of the sending domain and IP? — Receivers keep a running score based on engagement, complaints, and history. A new domain with no history starts at zero. A domain with prior spam complaints starts negative.
- Does the content match the signals? — Content filtering is real, but it is the last check, not the first. By the time filters reach content, the authentication and reputation signals have already done most of the work.
The key insight: If your email is going to spam, the fix is almost certainly in Layer 1, 2, or 4 of the deliverability stack — DNS, authentication, or reputation — not in your subject line or copy.
The most common causes, in order of frequency
1. Missing or broken authentication
If your domain is missing SPF, DKIM, or DMARC records — or if those records are misconfigured — receivers treat you as an unknown or suspicious sender. Gmail in particular has made DMARC alignment a hard requirement for bulk senders since early 2024.
Fix first: check your SPF, DKIM, and DMARC records. Every domain you send from needs all three.
2. Low domain or IP reputation
Reputation is the receiver's memory. It is built from: complaint rates (users hitting "this is spam"), hard bounce rates, spam trap hits, and engagement rates (opens, replies). A domain with a high complaint rate will send to spam regardless of how good the content is.
Check your domain reputation in Google Postmaster Tools. If it shows "bad" or "low," reputation is your problem.
3. Sending infrastructure shared with spammers
If you are using a shared IP pool — common with lower-tier ESPs — your reputation is partly determined by the other senders on that pool. A few bad actors on the same IP can drag your deliverability down even if you are doing everything right.
4. Too many sending sources, too few records
Modern SaaS stacks often have five, ten, or more systems sending email as your domain: your ESP, your CRM, your transactional email service, your support tool, your outbound sales tool. Each one needs to be authorised in your SPF record and signing with DKIM. Unauthorised sending sources look like spoofing.
5. Sending to unengaged or purchased lists
High complaint rates and low engagement rates are the most direct path to a poor reputation. Sending to purchased lists, to contacts who have not heard from you in years, or to anyone who didn't explicitly opt in will raise complaints and teach receivers that your mail is unwanted.
What content filters actually look at
Content filtering is real but it is rarely the primary cause of spam placement for legitimate senders. When content filtering does trigger, it is usually one of these:
- Spam-trigger phrases in combination with poor authentication and low reputation
- Images with very little text (a tactic spammers use to dodge text-based filters)
- Broken HTML or excessively complex code
- Links to domains with bad reputation
- A completely different sending IP or domain from your normal pattern
The subject line myth: Subject lines almost never cause spam placement on their own. A subject line with the word "free" or multiple exclamation marks is not going to spam if your authentication is clean and your reputation is good. Fix the fundamentals first.
How to diagnose your specific situation
- Check authentication: use MXToolbox or a similar tool to verify your SPF, DKIM, and DMARC records for every sending domain.
- Check reputation: open Google Postmaster Tools and look at your domain reputation and spam rate graphs.
- Check your sending sources: audit every system that sends email as your domain and make sure each is authorised.
- Run a deliverability seed test: services like GlockApps or Mail-Tester will show you which inboxes your mail is landing in and flag authentication issues.
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