Based on experience, one of the reasons emails end up in spam folders, even when following best practices, is misleading subject lines. Spam filters are designed to catch anything that looks deceptive, even if the email itself is legitimate. If a subject line sets an expectation that the content does not deliver, there is a good chance the email will get flagged.
I once ran a campaign with a subject line that said, "Your order is confirmed!" even though it was a promotional email offering a discount on future purchases. The intention was to grab attention, but it backfired. Spam filters flagged it because it looked like a transactional email but did not match the content. Worse, some customers got frustrated, thinking they had been charged for something they never ordered.
Email providers detect misleading subject lines by analyzing patterns in email content, user behavior, and past spam reports. Their filters compare subject lines with the actual email content, looking for inconsistencies. If a subject line suggests an order confirmation but the email lacks transactional details, such as an order number or shipping update, the system may classify it as suspicious and filter it out of the inbox.
Matt Little, Owner & Managing Director, Festoon House
Even when you follow all the best practices, emails can still land in spam, and one big reason is the reputation of the sending domain.
I'm a business owner and know that even if you—clean lists, clear opt-ins, great content—if the domain has a history of low engagement or spam complaints (even from years ago), email providers start flagging them. It's frustrating because fixing it isn't instant. It takes time to rebuild trust by improving open rates, reducing bounces, and getting more recipients to interact with your emails.
Also, I noticed that spam filters constantly evolve. What worked six months ago might not work now. Even small things, like certain trigger words, too many links, or an image-heavy email, can raise red flags. Sometimes, it's not even your content--it's how your emails are formatted or the technical setup, like missing authentication records (SPF, DKIM, DMARC). These back-end details may seem minor, but they make a big difference in whether an email gets delivered or tossed into spam.
Joseph Passalacqua, Owner & CEO, Maid Sailors
Even well-crafted emails can end up in spam if they contain too many elements that trigger automated filtering systems. It is not just about avoiding words like "free" or "guarantee"—modern spam filters analyze the structure of an email, including the balance of text to images, the number of links, and even how much personalization is included.
A common mistake is making emails look too much like advertisements. Emails that are mostly images with little text, have too many different fonts or include excessive links often get flagged. One way we avoid this is by keeping our emails clean and simple, using minimal images, and focusing on making them feel like personal messages rather than promotional blasts. A well-balanced email has a much better chance of making it to the inbox.
Jehann Biggs, President & Owner, In2Green
One key reason is recipient engagement (or lack of it). Even if you follow all the technical best practices, if people don't open, reply, or click on your emails, mailbox providers (like Gmail, Outlook) may assume your emails are unwanted and start marking them as spam.
A SaaS company sent weekly newsletters with useful content. They followed best practices like SPF, DKIM, DMARC authentication, and a clean email list. But still, many emails landed in spam. When they checked Google Postmaster Tools, they found low engagement rates—recipients weren't opening or clicking the emails.
How to Fix It:
By improving engagement, mailbox providers see your emails as valuable, reducing the chances of them going to spam.
Umair Ahmed Qureshi, SEO Specialist | Organic Growth Marketer | Content Marketing, FOCAL by Mozn.sa
Uncommon sender names can lower email credibility. I once experimented with sending marketing emails from "The Energy Savings Team" instead of using an actual person's name. Customers had subscribed to hear from an expert, but the vague sender name made emails look like generic promotions. Spam filters likely saw this as an attempt to mask the true sender.
Switching back to real names restored trust. Customers responded more to emails from "Ben at Electricity Monster" than from an abstract brand identity. Filters favor personal communication, and the open rate jumped by 30% once the sender matched customer expectations. A recognizable name builds familiarity, and familiarity keeps emails out of spam.
Benjamin Tom, Digital Marketing Expert and Utility Specialist, Electricity Monster
It often has to do with the email's content being flagged by spam filters. These filters look for certain patterns that might indicate a potential spam message, like the excessive use of links or certain trigger words that are commonly associated with scams. Heavy use of phrases like "limited-time offer" or "guaranteed results" can easily raise red flags.
I have observed this in my own work when communicating with clients and suppliers. If the email includes too many promotional terms, even in a legitimate context, it may get caught in spam filters. To avoid this, I try to keep the language natural and clear.
Adam Bushell, Director, AB Electrical & Communications
Sending images with embedded text instead of regular HTML has caused problems. I once tried using a visually polished email template with service reminders that included an image containing appointment details. The email looked great, but most customers never received it. Spam filters could not read the text inside the image and assumed it was a blank or misleading email. If an email lacks readable content, filters assume it is hiding something suspicious.
Switching back to plain text emails solved the issue. A simple confirmation message with clear text performed better than a fancy graphic. Customers appreciated the straightforward format, and delivery rates improved by 50%. Emails do not need to look like ads to be effective. Keeping things simple has ensured that service reminders arrive where they should.
Caleb John, Director, Exceed Plumbing
One key reason some emails still end up in spam folders, despite following best practices, is the lack of proper authentication protocols, specifically SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance).
These protocols help email providers verify that the emails are coming from a legitimate source and have not been tampered with during transmission. If your email domain is not properly authenticated, it's more likely that email providers will mark your message as suspicious and direct it to the spam folder. Even if your email content adheres to best practices (e.g., relevant subject lines, personalized content, and a clean design), poor authentication can still negatively affect deliverability.
Ensuring proper setup of SPF, DKIM, and DMARC not only helps improve email deliverability but also builds trust with email service providers (ESPs) like Gmail, Outlook, and Yahoo. This is a crucial step in email marketing to ensure your messages reach your recipients' inboxes.
Navin Kumar, Sales and Marketing (Digital Marketing), EDS FZE
Emails that mention too many dollar amounts in the subject line often get flagged. I learned this when trying to send bulk pricing estimates for large-scale weather protection structures. Including multiple cost breakdowns like "$50,000 - $200,000 solutions available" made filters suspicious. Large numbers, especially when grouped together, can mimic phishing attempts or scams. Even legitimate offers suffer from this issue. Rewording subject lines made a difference. Instead of listing price ranges, I referenced financing options or scheduling a consultation. The actual numbers remained in the email body, but without raising red flags at the inbox entry point. This simple change increased open rates by 35% and kept our proposals from vanishing into spam folders.
Barbara Robinson, Marketing Manager, Weather Solve
Sending emails at odd hours triggers filters more than people realize. I noticed a pattern when sending automated follow-ups at exactly midnight. The logic was to reach inboxes before competitors, but email providers treated this as bot-like behavior. Messages sent at unusual times, especially in high volumes, raise suspicion. Even legitimate emails get flagged when they look like automated mass messages with no human timing behind them.
Adjusting send times to normal business hours changed everything. Scheduling emails between 9 AM and 5 PM, when people are more likely to respond, improved deliverability by 40%. Automated does not mean random. Emails should mimic natural human behavior to avoid being marked as suspicious.
Nathan Mathews, CEO and Founder, Roofer
The biggest reason emails end up in spam folders is poor list hygiene. If your email list isn't kept clean, even the most well-crafted emails can get flagged. Old or inactive addresses, or sending too many emails at once, triggers spam filters. To avoid this, regularly update your list and ensure your emails are targeted to engaged subscribers. It's not just about what you're sending, it's about ensuring the people you're sending to actually want to hear from you.
Bennett Barrier, Chief Executive Officer, DFW Turf Solutions
Spam filters rely heavily on past behavior, and one of the biggest reasons emails end up in spam is a poor history of email-sending patterns. If a company suddenly goes from sending a few emails a month to blasting out thousands, it raises red flags. Email providers see that as suspicious, even if the emails themselves are legitimate.
The best way to avoid this is to warm up an email domain gradually. Instead of sending mass emails all at once, start small and build up over time. This signals to email providers that the sender is trustworthy and not suddenly spamming people. Consistency matters, so sticking to a regular sending schedule and avoiding sudden spikes helps keep emails in the inbox instead of the spam folder.
Mark Sanchez, Founder & Senior Real Estate Manager, Tropic Residential
Reputation plays a huge role in email deliverability, and one major issue that gets overlooked is shared IP addresses. Many businesses use email marketing platforms that send messages from shared servers, and if other users on that IP have been sending low-quality or spammy emails, it can impact everyone on that network. Even if an email follows best practices, a bad reputation from other senders can cause it to be filtered out.
A good way to fix this is to monitor sender reputation using tools like Google Postmaster Tools or invest in a dedicated IP if email volume is high enough. Sending consistent, high-quality emails also helps improve reputation over time. It is frustrating when emails land in spam for reasons beyond your control, but understanding how email providers judge sender reputation can make a big difference in getting messages through.
Linzi Oliver, Commercial Marketing Manager, HorseClicks
I'd point to one sneaky culprit here and that is inconsistent sender reputation tied to IP warmup mishaps. You can have your SPF, DKIM, and DMARC all lined up, but if you're sending from a fresh IP address that hasn't been properly warmed up—meaning you didn't gradually build a sending volume history with email providers like Gmail or Outlook—you're toast.
A cold IP blasting out 10,000 emails on day one, even with perfect authentication, screams "spam risk" to their algorithms. I learned this the hard way when we launched a new campaign for our espresso machine parts and saw open rates tank. We had to back off, start small with a few hundred sends, and scale up over weeks to build trust. It's not sexy, but it's the kind of practical fix I'd prototype in my shop—test, tweak, and iterate until it works.
Wes Wakefield, Founder & CEO, Pro Coffee Gear
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