Why are emails not appearing in the inbox despite ESP reporting successful delivery?
Matthew Whittaker
Co-founder & CTO, Suped
Published 8 May 2025
Updated 19 Aug 2025
6 min read
It can be incredibly frustrating to see your Email Service Provider (ESP) report that emails have been successfully delivered, yet your recipients insist they never received them in their inbox. This common discrepancy points to a subtle, yet critical, misunderstanding of what delivery truly means in the world of email. It's a widespread challenge that many senders encounter, leading to lost engagement and missed opportunities.
When your ESP confirms successful delivery, it typically means the receiving mail server responded with a 250 OK status. This SMTP (Simple Mail Transfer Protocol) response indicates that the receiving server has accepted the email for processing. However, this acceptance does not guarantee that the email will land in the recipient's primary inbox. Many layers of filtering and sorting occur after this initial acceptance, often without any further notification back to the sender or ESP.
Hidden filtering: why emails disappear after delivery
So, if the ESP says delivered, but the email isn't in the inbox, where could it be? The most common scenario is that the email was filtered into another folder, such as the spam (or junk) folder, or a categorized tab like Promotions or Updates, which is common with providers like Gmail. Recipient-side filters or client settings can also play a role, moving emails automatically or even silently deleting them if specific rules are triggered, as discussed in articles about delivered but missing emails.
Understanding successful delivery
An email is deemed 'delivered' by your ESP once the recipient's mail server acknowledges receipt with an SMTP 250 OK response. This means the message has been accepted at the receiving end of the connection.
The post-delivery journey
After acceptance, the email undergoes further internal scanning by the Mailbox Provider's (MBP) filters. This process determines whether the email goes to the inbox, spam folder, another categorized tab, or is silently dropped (also known as silent dropping). Your ESP has no visibility into these post-acceptance actions.
A primary factor is sender reputation. If your domain or IP address has a low reputation, mailbox providers will be more aggressive in filtering your emails. This can be influenced by spam complaints, low engagement rates, or being listed on a blacklist (or blocklist). A blocklist check should always be part of your routine checks. Another common issue is authentication failures. If your SPF, DKIM, or DMARC records are misconfigured, emails might be flagged as suspicious, even if they reach the receiving server. This often explains why emails land in spam despite passing authentication.
Beyond filtering: delays and recipient-side anomalies
Recipient-side filters
User rules: Recipients may have set up personal rules to move or delete emails from specific senders.
Mailbox full: While less common, a full mailbox can prevent new emails from appearing, even if accepted by the server.
Client settings: Certain email clients (like older Outlook versions) might have local display issues or caching problems.
Sender-side issues
Low engagement: If recipients consistently don't open or click your emails, MBPs can start filtering them. This is a common reason emails show as received but not in the inbox.
Content issues: Spammy keywords, excessive links, or poor formatting can trigger filters.
High complaint rates: If too many users mark your emails as spam, your reputation tanks, leading to filtering.
Sometimes, the issue isn't about filtering at all, but simply a delay. Mail servers can temporarily defer email delivery due to network congestion, rate limits imposed by the recipient server, or even greylisting. In these cases, your ESP will usually retry sending the email, and it might eventually arrive, just later than expected. This can be particularly true for emails delayed with no errors in your logs.
Recipient server issues, although less common, can also lead to messages being accepted but never reaching the user. This is often the case with email showing as delivered but not in inbox in some enterprise environments, where complex internal routing or aggressive firewall settings might be at play. Sometimes, these issues are specific to certain providers, such as SendGrid emails not appearing in Outlook inboxes.
Diagnosing the problem: what to check
When troubleshooting, always start by confirming with the recipient. Ask them to check their spam or junk folders, as well as any other tabs or categories their email provider might use (e.g., Promotions, Social). Also, check if they have any personal rules set up that might be diverting or deleting your emails.
Next, dig deeper into your ESP's logs. While they might report delivered, some providers offer more granular data, such as details about the SMTP conversation or any deferred retries. This can sometimes give clues about temporary issues or specific rejections by the recipient's server that don't result in a hard bounce.
Directly impacts delivery to recipient servers, often leading to rejection.
Long-term solutions for inbox placement
Improving your overall email deliverability is a continuous effort. Focus on building and maintaining a strong sender reputation. This involves consistently sending relevant and engaging content to an active and permission-based list. Avoid sending to old or unengaged contacts, as this can quickly degrade your reputation.
Regularly clean your email lists to remove invalid, inactive, or unresponsive addresses. Incorporate explicit opt-in processes and make it easy for subscribers to manage their preferences or unsubscribe. A clean list reduces bounces and spam complaints, both of which negatively impact your sender reputation and can lead to emails disappearing from the inbox. This proactive approach is key to improving deliverability in 2025.
Beyond technical configurations, ensure your email content is high-quality and free of spam triggers. Personalize messages where possible, and include clear calls to action. Encourage recipients to add your email address to their contacts or mark your emails as not spam, as these positive interactions signal to mailbox providers that your mail is desired.
Views from the trenches
Best practices
Continuously monitor your domain and IP reputation using tools like Google Postmaster Tools.
Implement strong email authentication: SPF, DKIM, and DMARC are non-negotiable for deliverability.
Maintain a clean, engaged email list by regularly removing inactive or invalid addresses.
Send consistent email volume to avoid sudden spikes that can trigger spam filters.
Encourage positive engagement: ask recipients to reply, open, click, and add you to contacts.
Common pitfalls
Assuming 'delivered' means 'inbox' without further investigation.
Ignoring low engagement rates, which negatively impact sender reputation over time.
Sending to unengaged or old lists, increasing spam complaints and silent drops.
Not checking blocklists (blacklists) regularly; being listed can halt delivery entirely.
Failing to monitor DMARC reports for authentication and delivery insights.
Expert tips
If emails are going to Gmail, check their Postmaster Tools for spam rates and reputation.
For individual recipient issues, ask them to check all folders, including promotions, social, and spam.
Verify the recipient's email address by sending a manual test email outside your ESP.
Be aware that some providers, like Hotmail (now Outlook.com), are known for aggressive filtering and silent dropping.
Delays can occur due to greylisting or rate limits; monitor for patterns.
Expert view
Expert from Email Geeks says the 250 OK response from a mail server indicates acceptance, but not necessarily inbox placement, as subsequent filtering occurs outside the ESP's visibility.
2018-12-03 - Email Geeks
Marketer view
Marketer from Email Geeks suggests checking if internal segmentation rules, such as those based on engagement, might be excluding recipients from receiving certain emails.
2018-12-03 - Email Geeks
The path to better inbox placement
The distinction between an ESP reporting successful delivery and an email actually landing in the recipient's inbox is crucial for effective email marketing and communication. While your ESP confirms acceptance by the receiving server, the journey doesn't end there. Mailbox providers employ sophisticated filtering mechanisms, and various factors, from sender reputation to recipient engagement, can influence whether your email reaches its intended destination within the inbox or is diverted elsewhere.
By understanding these nuances, diligently troubleshooting, and consistently applying best practices in email authentication, content, and list hygiene, you can significantly improve your inbox placement rates. It requires a holistic approach to email deliverability, continuously adapting to the evolving landscape of mailbox provider algorithms and user behaviors. Regularly checking DMARC monitoring and maintaining your domain reputation are ongoing tasks that will yield positive results.
Gmail. Recipient-side filters or client settings can also play a role, moving emails automatically or even silently deleting them if specific rules are triggered, as discussed in articles about delivered but missing emails.
using Google Postmaster Tools
