Every day, hundreds of billions of emails are sent, and nearly half of them are spam. Inbox providers like Gmail, Outlook, and Yahoo must block unwanted messages while ensuring legitimate ones reach the inbox.
For marketers, this means your campaigns must be wanted, expected, and compliant with evolving standards. Deliverability is more than just “sent,” it’s whether your message actually lands in the inbox instead of the spam folder.
At the heart of deliverability is your sender reputation, a trust score based on your past sending behavior. A high reputation means better inbox placement and more opportunities for conversion. A low reputation can result in spam filtering, blocking, or even blacklisting.
Sender Reputation: Your Email Credit Score
Inbox providers evaluate senders much like a credit bureau scores borrowers. The stronger your sending history, the more likely your emails are delivered. Once damaged, reputation is much harder to repair than maintain.
Key factors that impact sender reputation include:
- Spam Complaints: Recipients clicking “Spam” or “Junk.”
- Hard Bounces: Sending to invalid or expired email addresses.
- Spam Traps: Addresses designed to catch senders using poor-quality or non-permissioned data.
- Engagement: Low open or click rates.
- Content Blocks: Trigger words, blacklisted links, or poor link reputation.
- Sending Patterns: Sudden spikes or erratic volume.
- Authentication: Missing or misconfigured SPF, DKIM, and DMARC.
- Third-Party Blacklists: Listings with services like Spamhaus.
Reputation tracking is fragmented, there’s no central score. Each inbox provider maintains its own records, and they often won’t disclose complaint or trap data. Monitoring and prevention are your best tools.
Three Pillars of Strong Deliverability
Your sender reputation and inbox placement depend on three interconnected areas, all of which must be addressed together.
1. Lead Generation: Build Permission-First Lists
High-quality, opt-in lists are the foundation of deliverability.
Best Practices:
- Get Explicit Permission: Consent must be freely given, specific, informed, and unambiguous. Avoid pre-checked boxes or implied consent from events or purchases.
- Avoid Purchased or Shared Lists: They often contain spam traps and non-engaged addresses.
- Use Double Opt-In: Confirm subscribers’ identity and intent before adding them to your marketing list.
- Set Expectations: Clearly state what you’ll send, how often, and from what domain or brand.
- Limit Role-Based Addresses: Emails like info@ or sales@ are often shared and more prone to complaints unless explicitly opted in.
2. Sending Practices: Consistency, Compliance, and Content
Even the best list can fail if your sending habits damage trust.
Best Practices:
- Authenticate Your Emails: As of February 2024, Gmail and Yahoo require bulk senders to have SPF, DKIM, and DMARC records configured. Without these, your emails may be rejected.
- Meet Subscriber Expectations: Keep content, frequency, and branding consistent. If you make major changes, notify subscribers, and re-confirm permission if needed.
- Segment Your List: Send targeted, relevant content to improve engagement.
- Write Honest Subject Lines: Avoid misleading or “spammy” phrasing.
- Enable One-Click Unsubscribe: Required for bulk senders by major inbox providers.
- Stay Compliant: Follow CAN-SPAM, CASL, GDPR, and any applicable state laws.
- Design for Deliverability: Maintain a balanced text-to-image ratio, avoid spam trigger words, and ensure mobile-friendly layouts.
3. List Maintenance: Keep It Clean and Engaged
A clean, engaged list protects your reputation and improves ROI.
Best Practices:
- Shorten the Engagement Window: Consider re-engaging or removing subscribers after 90 days of no opens or clicks (versus the old six-month standard).
- Remove Hard Bounces Immediately: Invalid addresses hurt your reputation.
- Validate Regularly: Detect and remove spam traps and outdated contacts.
- Don’t Revive Dormant Lists: Permission fades; if you haven’t emailed a list in 6+ months, treat them as new and reconfirm consent.
Troubleshooting Deliverability Issues
When messages don’t appear in the inbox, the problem usually falls into one of three categories:
- Not Sent: Suppressed addresses, automation errors, or campaign timing issues.
- Not Delivered: Bounced due to reputation, invalid addresses, authentication failure, or technical blocks.
- Delivered but Filtered: Routed to spam due to content, engagement patterns, or manual filters.
Bounce Code Clues:
- Authentication Issues:
550 Message rejected due to sender’s DMARC policy
550 Sender did not meet SPF rules
- Reputation Blocks:
554 Refused. Your domain name is blacklisted.
- Spam Content:
554 Message blocked due to spam content
552 URL in this message is blacklisted
- Recipient Issues:
552 User’s mailbox is full
550 Email account does not exist
Proactive Monitoring Tools:
- Google Postmaster Tools
- MXToolbox
- Validity / Everest
- ISP Feedback Loops
Deliverability Checklist
Before you hit “send”, confirm:
- SPF, DKIM, and DMARC are configured and passing.
- All recipients have documented, explicit consent.
- Content is relevant, mobile-friendly, and free of spam triggers.
- Inactive contacts are segmented or removed after 90 days.
- One-click unsubscribe is enabled and visible.
- Sending volume matches your normal patterns.
- Engagement metrics are healthy.
Final Takeaway
Email deliverability is a moving target shaped by inbox provider rules, technical standards, and subscriber behavior. The best way to stay in the inbox is to send only to people who want your messages, deliver content they expect, maintain a healthy list, and stay ahead of compliance and authentication requirements.
A trusted sender is a successful sender. When every email is both wanted and expected, you’ll see better deliverability, stronger engagement, and higher ROI.
