Over 160 billion spam emails are sent daily, making up nearly 47% of global email traffic. The sheer volume of spam has forced email providers to tighten their filters, making it harder for legitimate emails to reach inboxes, even when they follow the rules.
If your emails are landing in spam, it’s not random. Gmail, Outlook, Yahoo, and other providers analyze multiple factors before deciding where your email belongs. If you’re not monitoring these factors, you will likely lose visibility, engagement, and potential customers.
Email providers don’t just block obvious spam anymore. They now use advanced filtering systems that analyze who is sending the email, what’s inside, and how people react to it before deciding if it belongs in the inbox.
The reason? Spam is still a massive problem. In 2024:
Nearly 47% of all emails sent worldwide are spam. That’s almost one in every two emails.
Over 3.4 billion phishing emails are sent daily, making fraud protection a top priority.
The U.S. alone sends more than 7.8 billion spam emails per day, leading to strict monitoring by email providers.
Spam filters have moved beyond simple keyword detection. Avoiding phrases like “free money” or “act now” is no longer enough. Today, email providers analyze sender reputation, email traffic patterns, and user behavior to decide whether an email is safe.
Even legitimate emails can get flagged. Mass emails from new senders, sudden spikes in volume, or unverified domains often appear suspicious. If an email seems risky—even if it isn’t spam—it may never reach the inbox.
Understanding these filters is key to improving deliverability. The next sections will explain why emails get flagged and how to fix them.
Email providers assess the sender's reputation to decide whether an email reaches the inbox or spam folder. A low domain reputation signals that your emails may not be trustworthy, reducing deliverability.
Several factors hurt the sender's reputation:
Spam complaints: Too many recipients marking emails as spam leads to automatic filtering.
High bounce rates: Sending to invalid or inactive addresses signals poor list management.
Low engagement: Emails that aren’t opened, clicked, or replied to indicate a lack of interest, lowering inbox placement.
To monitor your reputation, use Google Postmaster Tools or SenderScore. If it's low, focus on removing unengaged contacts, improving email relevance, and sending consistently to an engaged audience.
Without authentication records, email providers cannot verify that an email was legitimately sent by the claimed sender, increasing the risk of it being filtered out.
The three most important authentication protocols are:
SPF (Sender Policy Framework): Specifies which mail servers are authorized to send emails on behalf of a domain.
DKIM (DomainKeys Identified Mail): Digitally signs emails to prevent tampering and confirm authenticity.
DMARC (Domain-based Message Authentication, Reporting & Conformance): Instructs email providers on handling messages that fail SPF or DKIM checks.
Tools like MXToolbox can check DNS records to verify authentication status. If authentication is missing, it should be configured within the domain’s settings to ensure email legitimacy.
Spam filters analyze email content for patterns associated with unwanted messages. While modern filtering systems focus more on sender behavior than keywords, specific formatting and wording choices still raise red flags.
Content-related issues that can cause emails to be flagged include:
Excessive use of promotional language: Phrases like “free,” “limited-time offer,” or “guaranteed” can trigger filters.
Overuse of images with little text: Emails that contain mostly images and minimal text are often flagged.
Misleading subject lines: Using all caps, excessive punctuation, or deceptive wording reduces trust.
Link shorteners and too many hyperlinks: Bitly or TinyURL links are frequently abused by spammers and can lower deliverability.
Running emails through spam testing tools like Mail-Tester before sending can help identify potential issues.
Email providers monitor sending frequency and patterns to detect suspicious behavior. If a domain suddenly sends a large volume of emails from an IP that was previously inactive, it may be marked as spam.
Suspicious sending behaviors include:
Sending bulk emails from a newly created domain: New domains lack an established reputation and are more likely to be filtered.
Irregular sending patterns: Large spikes in email volume can indicate spam-like behavior.
Sending to unengaged recipients: If many emails go unopened, future messages may be deprioritized.
A gradual warm-up process is recommended for new sending domains. This involves starting with smaller email batches and gradually increasing volume to establish a positive reputation.
Spam complaints directly impact deliverability. If too many recipients mark emails as spam, email providers will flag your domain, filter future messages, or even block them entirely.
A healthy spam complaint rate should stay below 0.1%—meaning no more than one complaint per 1,000 emails sent.
Spam Complaint Rate Formula:
For example, if 75 people report spam out of 50,000 emails sent:
(75/50,000)×100=0.15%
A rate above 0.1% can damage your sender's reputation, leading to worse inbox placement, higher filtering, and potential blacklisting.
Monitoring spam complaints through Google Postmaster Tools, ESP dashboards, or feedback loops helps identify patterns and adjust email strategies before they harm deliverability.
If your domain or IP is blacklisted, first check your status using tools like MXToolbox or Spamhaus. Most blacklists have a delisting process, which may involve submitting a request or demonstrating improved sending practices. Before requesting removal, fix any issues that led to blacklisting, such as high spam complaints, poor authentication, or sending bulk emails too aggressively.
If legitimate emails are landing in spam, mark them as “Not Spam” or “Not Junk” in your email client to help train the filter. Adding the sender’s email address to your contacts or safe sender list can prevent future emails from being misclassified. For businesses, regularly checking spam folders ensures you don’t miss important client communications.