Why are emails not appearing in the inbox despite ESP reporting successful delivery?

Key findings

• SMTP 250 Response: An ESP's 'successful delivery' status means the recipient's mail server provided a 250 Ok response, signifying acceptance of the email, not necessarily inbox placement.
• Spam Folder Placement: A common reason emails are 'delivered' but not seen is that they were filtered directly into the spam or junk folder.
• Silent Drops: In rare instances, particularly with low sender reputation or poor engagement, ISPs (like Gmail) might silently drop emails without generating a bounce.
• Recipient-Side Filtering: Individual user settings, security software, or corporate firewalls on the recipient's end can also quarantine or redirect emails before they reach the primary inbox.

Key considerations

• Check Spam/Junk Folders: Always advise recipients to check their spam or junk folders, as emails frequently land there. This is often a first troubleshooting step to diagnose deliverability issues.
• Monitor Engagement: Low engagement rates can signal to ISPs that your emails are not valuable, leading to future filtering. This is a common reason emails go to spam.
• Use Seed Lists: Employ seed lists or test accounts across various email providers to get real-time insight into where your emails are actually landing. This helps verify your email's journey post-delivery.
• Verify Authentication: Ensure your domain has correctly configured SPF, DKIM, and DMARC records to build trust and prevent legitimate emails from being flagged.

Key opinions

• Misleading Delivery Reports: Marketers frequently find that 'delivered' simply means the email was accepted by the recipient's server, not that it landed in the primary inbox.
• Spam Folder Diversion: A common cause for missing emails, even with 100% deliverability, is that they are routed directly to the spam folder, particularly in services like Gmail.
• Engagement-Based Filtering: Lack of interaction from recipients can lead email providers to filter future emails into spam or potentially discard them without a bounce, a behavior observed particularly with Gmail.
• IP Warming Challenges: During IP warming, marketers often report experiencing good 'deliverability' but poor inbox placement, as new IPs build reputation and are cautiously filtered. This is a key aspect of email deliverability during IP warming.
• Manual Checks Required: Marketers resort to manually checking test accounts and sending individual emails to verify actual delivery and inbox placement.

Key considerations

• Test Account Verification: It is crucial to verify actual inbox placement by checking test accounts directly, as ESP delivery metrics can be misleading.
• Review Segmentation: Confirm that segmentation rules are not inadvertently excluding or treating test accounts differently.
• Proactive Spam Folder Checks: Consistently check spam or junk folders for missing emails, as they are a frequent destination. This is key to improving overall email deliverability.
• Recipient-Side Troubleshooting: Consider potential recipient-side issues such as security software, firewalls, or user settings that might be quarantining emails.

Key opinions

• 250 Response is Acceptance: The 250 SMTP response signifies that the recipient server has accepted responsibility for the message, but it gives no indication of where it will land within the mailbox.
• Post-Acceptance Filtering: After an email is accepted, mailbox providers implement internal filters that can direct emails to spam, junk, or other categorized folders, or even delay delivery.
• Silent Drops by ISPs: Although rare, certain ISPs, like Gmail, may occasionally drop emails silently, meaning they accept the message but then discard it without generating a bounce notification, often due to very low sender reputation or engagement. This contributes to Gmail delivery delays and spam placement issues.
• Hotmail/Outlook Specifics: Hotmail (now Outlook.com) is noted for its aggressive filtering, sometimes dropping emails or placing them in junk without explicit bounce notifications. This is a common issue for Hotmail email deliverability.
• Cloudmark Protection: Domains protected by Cloudmark may drop emails if the end-user has not explicitly created a spam folder.

Key considerations

• Sender Reputation Management: Maintaining a strong sender reputation is paramount. ISPs use this to determine where emails land, impacting their decision to filter or deliver to the inbox.
• Direct Testing: If emails are not appearing, send a manual email from a standard mail client to the problematic address to confirm if any delivery occurs outside the ESP's system.
• Inbox Placement Testing: Regularly use seed lists to monitor where your emails land across major ISPs, as deliverability metrics alone are insufficient.
• Authentication Standards: Ensure proper SPF, DKIM, and DMARC alignment. These authentication methods are crucial for building trust with ISPs and reducing the chance of emails being filtered or dropped.

Key findings

• SMTP Protocol Standards: RFC 5321 (SMTP) defines the 250 status code as 'Requested mail action okay, completed,' indicating the receiving SMTP server has accepted responsibility for the message, but it makes no guarantees about where the email will be placed within the recipient's mailbox.
• Post-Acceptance Processing: Documentation from major mailbox providers (like Google and Microsoft) explains that after an email is accepted, it undergoes internal filtering processes to determine its final placement, which can include the inbox, spam folder, or other categories.
• Filtering Algorithms: Mailbox providers use complex, often AI-driven, algorithms that evaluate sender reputation, content, and user engagement signals to classify incoming emails and determine their inbox placement.
• Authentication Importance: Official guidelines emphasize that proper email authentication (SPF, DKIM, DMARC) is fundamental for a sender's legitimacy and significantly influences how recipient servers trust and process emails, impacting whether they are filtered or delivered to the inbox.

Key considerations

• Beyond Transport: Marketers must differentiate between successful email transport (delivery to the server) and actual inbox placement, as documented by ISP policies.
• Reputation is Key: According to provider documentation, maintaining a positive sender reputation is critical for consistently reaching the primary inbox, reducing the likelihood of emails being diverted to spam.
• Authentication Standards: Adhering to and implementing robust authentication like DMARC, SPF, and DKIM is explicitly recommended to prevent emails from being flagged or dropped.
• Content and Engagement Guidelines: Documentation often provides guidelines on email content and engagement best practices to help senders achieve optimal inbox placement and avoid common pitfalls leading to filtering.