You wrote a great email. You sent it to your whole list. Then a customer tells you it landed in their spam folder. It feels unfair, but inbox providers like Gmail, Outlook and Yahoo are not guessing. They follow signals, and when enough signals look bad, your email goes to junk.
The good news is that most of these signals are in your control. Here are the main reasons and how to fix them.
1. Your domain is not authenticated
Email has three simple checks called SPF, DKIM and DMARC. Together they prove that an email really comes from you and was not faked by someone else.
If these are missing, inbox providers have no way to trust you. Gmail, Yahoo and Microsoft now expect bulk senders to have them in place, and emails without them are much more likely to be filtered or rejected.
Fix: set up all three for the domain you send from. Your email tool will show you the exact records to add. Our guide to SPF, DKIM and DMARC explains each one in plain words.
2. You are sending to people who did not ask
Bought lists, scraped contacts and old event lists are the fastest way to the spam folder. People who never signed up will mark you as spam, and every complaint hurts your reputation.
Fix: only email people who clearly agreed to hear from you. If you have an old list, send a short "do you still want to hear from us?" email and keep only those who reply or click.
3. Too many people mark you as spam
Inbox providers watch how many people hit the spam button on your emails. Google recommends keeping your spam complaint rate below 0.1 percent and never letting it reach 0.3 percent. That is just one or three complaints for every thousand emails.
Fix: make unsubscribing easy. A visible unsubscribe link is far better than a spam complaint. Also remind people why they are getting your email, for example "You are getting this because you signed up at our shop."
4. Your list is full of dead addresses
When you send to addresses that no longer exist, they bounce. Lots of bounces tell inbox providers that you do not look after your list.
Fix: remove hard bounces right away. Most email tools do this for you. Every few months, also remove people who have not opened or clicked anything in a long time.
5. People ignore your emails
Inbox providers also watch what people do with your emails. If most of your list never opens them, providers start to think your emails are not wanted.
Fix: send fewer, better emails. Ask yourself before every send: would I be glad to get this? Segment your list so people only get emails that match their interests.
6. Your content looks spammy
Content matters less than it used to, but some habits still raise flags:
- ALL CAPS subject lines and lots of exclamation marks.
- An email that is just one big image with little text.
- Link shorteners and lots of links to different websites.
- Phrases that sound like scams, such as promising guaranteed money.
Fix: write like a normal person writing to a customer. Use real text, a few clear links and a balanced mix of words and images.
7. You suddenly send a lot more
If you usually send 200 emails and suddenly send 20,000 from a new account or domain, it looks suspicious.
Fix: grow slowly. Start with your most engaged contacts and increase the size of your sends over a few weeks. This is called warming up your domain, and our warm up guide shows how to do it step by step.
How to check where you stand
- Send test emails to Gmail, Outlook and Yahoo accounts and see where they land.
- If you send a lot to Gmail users, sign up for Google Postmaster Tools. It shows your spam rate and domain reputation.
- Watch your email tool's reports for jumps in bounces, complaints or unsubscribes.
The short version
Authenticate your domain, email only people who asked, make leaving easy and send things people want to read. Do those four things consistently and the spam folder becomes a rare visitor rather than a regular one.
Mailbox providers update their filters and requirements regularly. This guide was last reviewed on August 4, 2026. Spotted something out of date? Tell us.