A client once told us their open rate dropped from 28% to 6% in a single week. Nothing about their content changed. What changed was their sending domain's reputation, and it happened because of one missing DNS record. Deliverability problems rarely come from one dramatic mistake. They build up from small technical gaps and content habits that inbox providers quietly flag over time.
Here are the seven most common causes, ranked from most frequent to least frequent based on what typically trips up small business senders.
1. Missing or Misconfigured SPF, DKIM, and DMARC Records
This is, in our experience, the single most common reason legitimate emails end up in spam. SPF, DKIM, and DMARC are authentication protocols that tell inbox providers your email actually comes from who it says it comes from. Without them, Gmail and Outlook have no way to confirm you're not an impersonator.
If you switched email service providers recently, added a new sending domain, or never set these up in the first place, this is the first thing to check.
How to check it:
- Use a free SPF/DKIM/DMARC checker tool (most email service providers have one built in).
- Confirm your DKIM signature is passing, not just present.
- Set DMARC to at least "p=none" for monitoring, then move to "p=quarantine" once you confirm legitimate mail passes.
Most email service providers walk you through this during setup, but the records need to be added to your domain's DNS settings by you or whoever manages your website hosting. This step gets skipped constantly because it feels technical and unrelated to "marketing."
2. Sending From a Brand-New Domain or IP With No History
Inbox providers trust senders with a track record. A brand-new domain has none, so it starts with a neutral or cautious reputation. If you immediately blast a large list from day one, spam filters treat that volume as suspicious.
This is sometimes called "warming up" a domain, and skipping it is a common mistake for businesses that just switched platforms or launched a new brand.
A simple warm-up approach:
- Send to your most engaged subscribers first (people who opened your last several emails).
- Start with smaller batches, a few hundred emails a day, and increase gradually over two to three weeks.
- Avoid sending your full list at once until your domain has a sending history.
3. Subject Lines and Content That Trip Spam Filters
Spam filters scan for patterns, not just keywords, but certain habits still raise red flags. Overuse of urgency language, excessive punctuation, or all-caps phrases can push a message toward spam, especially when combined with other risk factors like low engagement or new sending infrastructure.
Common culprits include:
- Subject lines with words like "free," "guarantee," or "act now" stacked together
- Excessive exclamation points or dollar signs
- ALL CAPS SUBJECT LINES
- A subject line that doesn't match the actual email content
Before: "FREE!!! Limited Time Offer, Act Now Before It's Gone!!!" After: "Your discount ends Friday"
The second version says the same thing without tripping the same filters. It's shorter, specific, and doesn't rely on punctuation to create urgency.
4. Poor List Hygiene: Old, Purchased, or Unverified Contacts
Sending to a list full of dead addresses, typos, or people who never actually opted in is one of the fastest ways to damage sender reputation. Every bounce and spam complaint counts against you, and purchased lists are almost always full of both.
Signs your list needs cleaning:
- Bounce rates above 2 percent
- Spam complaint rates above 0.1 percent
- Large chunks of subscribers who haven't opened anything in six months or more
- Addresses that look auto-generated or obviously fake
What to do about it:
- Run a list cleaning or email verification service before your next send if you haven't sent in a while.
- Remove hard bounces immediately; most email service providers do this automatically, but check your settings.
- Never buy or rent an email list. It feels like a shortcut, but it typically creates more deliverability damage than it's worth.
5. Low Engagement Dragging Down Your Sender Reputation
Gmail, Outlook, and other major providers track how recipients interact with your emails: opens, clicks, replies, deletions without opening, and "mark as spam" clicks. If a large share of your list ignores your emails or deletes them unread, that behavior signals to the inbox provider that your mail isn't wanted, even if no one actively marks it as spam.
This is why a smaller, engaged list often outperforms a larger, stale one. Sending to 2,000 people who open and click beats sending to 20,000 people who never touch your emails.
A basic re-engagement approach:
- Segment out subscribers who haven't opened anything in 90 to 180 days.
- Send this group a short, direct "Do you still want to hear from us?" email.
- Remove anyone who doesn't respond or re-engage within a couple of weeks.
This feels counterintuitive because you're shrinking your list on purpose, but it typically improves deliverability for everyone else on it.
6. A Broken or Buried Unsubscribe Process
When people can't find an easy way to unsubscribe, many of them click "report spam" instead. It's faster for them, and it's far more damaging for you. A single spam complaint does more harm to your sender reputation than dozens of clean unsubscribes.
Common mistakes here include:
- Unsubscribe links buried in tiny gray text at the very bottom
- A multi-step unsubscribe process that requires logging in or confirming multiple times
- No unsubscribe link at all (which also typically violates email marketing laws like CAN-SPAM or GDPR, depending on your audience)
Make the unsubscribe link visible, make it work in one click, and honor it immediately. It costs you a subscriber, but it protects your ability to reach everyone else.
7. Heavy Formatting: Too Many Images, Links, or Attachments
This one is lower on the list because it typically only causes problems when combined with the issues above, but it's still worth checking. Emails that are a single large image with almost no text, emails packed with more than a handful of links, or emails with attachments can all get flagged by spam filters, since these are patterns spammers use too.
Simple formatting fixes:
- Keep a healthy balance of text to images. An email that's 90 percent image and 10 percent text looks suspicious to filters.
- Limit links to what's necessary. A newsletter with 15 links to different products looks more like a spam blast than a personal message.
- Avoid attachments in marketing emails entirely. Link to a hosted file instead.
Quick Deliverability Checklist
If your emails are landing in spam, work through these in order, since the first ones on this list are the most likely culprits:
- Verify SPF, DKIM, and DMARC are set up and passing.
- Confirm your domain or IP has a sending history, or warm it up gradually if it's new.
- Rewrite subject lines that lean on urgency, all-caps, or excessive punctuation.
- Clean your list of bounces, unengaged contacts, and unverified addresses.
- Re-engage or remove subscribers who haven't opened anything in the last few months.
- Make your unsubscribe link visible and instant.
- Balance text and images, and cut down on excessive links.
Most deliverability problems are a combination of two or three of these, not just one. Start with authentication and list hygiene since they typically cause the biggest jumps in inbox placement, then work down the list from there.