Emails Going to Spam: The Causes in Order of Importance
Most people look for faults in the email text when something lands in spam. The cause is more often the domain, the reputation and the volumes. Here is the order to troubleshoot in.
Why do emails end up in spam?
Emails going to spam usually means the recipient's system does not trust the sender. The causes, in descending order: missing or incorrect domain authentication, poor reputation, volumes that are too high, addresses that do not exist and, last of all, the content. The text itself weighs least, even though it is what people tend to change first.
That is the opposite of what most people expect. When an important send disappears, you start scrutinising the subject line, swapping a word, removing a link. It feels concrete. But the filter has often made up its mind before it even reads the text.
So work through the causes from the top down. That way you avoid spending an afternoon rewriting an email that never had a chance.
1. Domain authentication: the first thing the filter looks at
Imagine someone calls from an unknown number and says they are your bank. You would want to be able to check that. The recipient's mail system does the same thing, and the check relies on three DNS records on your domain.
- SPF is a list of which servers are allowed to send email on the domain's behalf.
- DKIM is a digital signature showing that the email has not been altered along the way and that the sending domain stands behind it.
- DMARC tells the recipient what to do when SPF or DKIM fails, and ties the checks to the address shown in the From field.
The most common mistake is not that the records are missing altogether. It is that they are incomplete. You set up SPF for Google Workspace, but then someone connected a newsletter tool or a CRM that also sends email from the same domain, and that tool is not on the list. Some of your emails pass the check and the rest do not. It is hard to spot, because the emails you send by hand look fine.
Gmail and Yahoo also require authentication, including DMARC, from anyone sending at larger volumes. Skipping this step is therefore a bad idea even if you are below those thresholds. We go through how to set up the records, step by step, in our guide to SPF, DKIM and DMARC.
A simple test that needs no tools: send an email from your usual address to a Gmail account, open it and choose Show original. At the top you will see SPF, DKIM and DMARC with a result of PASS or FAIL. Anything other than PASS on all three is your first fix.
2. Reputation: why do some emails land in spam while others get through?
This is the question behind many searches, and the answer is that every sending domain has a reputation. Receiving systems build it from what has happened before: how many people have marked your emails as spam, how many have bounced, how many have replied, and whether you suddenly start sending far more than before.
That is why two colleagues at the same company can get completely different results. One sends ordinary quotes to customers who reply. The other sends cold emails to people who have never heard of the company. If they share a domain, they share a reputation, and the second person's sends drag down the first person's quotes.
Two things follow from this. Never send prospecting emails from the domain and inboxes you use for customers and referrals. And a brand-new domain has no reputation at all, which filters treat with suspicion. It needs to be warmed up, meaning built up gradually with a little traffic and real replies, before it can carry any volume.
3. Emails going to spam because you send too much, too fast
Picture an inbox that sends ten emails a day for several weeks and then, one Tuesday, four hundred. Nothing in the content needs to be wrong. The pattern is enough. It looks like a hijacked account or a mass mailing, and the filter reacts accordingly.
This is how we work in our own sends: dedicated domains that are warmed up in advance, few emails per inbox per day, short three-step sequences and never mass mailings. It is slower on paper. But a burned domain costs more time than it saves, and it is easier to keep a good reputation than to build a new one.
If you are wondering whether cold email would work in your particular market, you can book a free 30-minute call. Among other things, we go through how big the market is and check your SPF, DKIM and DMARC.
4. Addresses that have not been verified
If you send to an address that does not exist, you get a bounce back. A single bounce does not matter. But a high share of bounces tells receiving systems that you send without knowing who you are sending to, which is exactly what disreputable senders do.
So verify every address before it gets an email. Pause your sends immediately if bounces rise, and remove the addresses that have bounced so they do not get a second attempt. It is dull work, but it is cheaper than repairing a ruined reputation.
5. Content: important, but last in line
The lists of banned words, such as "free" and "guaranteed", are mostly superstition. Modern filters weigh many signals at once, and a single word rarely decides anything. What can do damage is patterns:
- many links, or links via URL shorteners
- images, attachments and tracking pixels in a first email
- exactly the same text sent to many recipients
- a subject line that promises something the email does not deliver
A short plain-text email with one clear point and a personal connection to the recipient usually does fine. It is also better writing. We describe how to structure it in our guide to B2B cold email.
Why do emails go to spam in Outlook?
Outlook and Microsoft 365 have their own filters, and many companies add rules set by their IT administrator on top. The causes are still the same as above: authentication, reputation and volume.
If you are on the receiving side, it is quick to fix. Mark the email as Not junk and add the sender to your safe senders. If you are on the sending side, that helps less, because you cannot ask a thousand recipients to do it. Then your own configuration is what counts. And Gmail's Promotions tab is not spam, but it gives a personal email just as poor visibility. A short plain-text email lands in the main inbox more often.
How to fix emails going to spam
- Check SPF, DKIM and DMARC using Show original in Gmail. Fix every FAIL, and add every tool that sends in your domain's name.
- Stop or reduce sending until authentication works. Pushing more emails into a broken setup makes the damage worse.
- Review reputation and bounces: how many have bounced, how many have replied. Switch off addresses that bounce.
- Cut the volume per inbox per day and build it back up gradually.
- Only then: shorten the text, and remove attachments and extra links.
Start by sending a test email from the address you prospect with to your own Gmail account, and read the result under Show original. If it says PASS on all three, the fault is probably further down the list. If it says FAIL anywhere, you have found it.
Frequently asked questions
Why do emails go to spam in Outlook?
Outlook and Microsoft 365 have their own filters, and many companies add their own rules on top. The causes are usually weak authentication, a poor reputation or volumes that rise too quickly. As a recipient, you can mark the email as Not junk and add the sender to your safe senders.
Does the content of an email affect whether it lands in spam?
Yes, but less than most people think. Individual words rarely decide anything. Many links, attachments, tracking images and identical text sent to many recipients weigh more. Check authentication, reputation and volume first.
How do I check whether SPF, DKIM and DMARC are working?
Send an email to a Gmail account, open it and choose Show original. At the top you will see the results for SPF, DKIM and DMARC. All three should say PASS.