Why are opted-in customers not receiving emails from my marketing campaign?
Michael Ko
Co-founder & CEO, Suped
Published 21 May 2025
Updated 17 Aug 2025
8 min read
It can be incredibly frustrating when you have a list of engaged, opted-in customers, but your marketing emails just aren't reaching their inboxes. You've confirmed they're subscribed, they haven't hard or soft bounced, and yet, reports from your sales team indicate that many are simply not seeing your messages.
This scenario is particularly common in B2B environments, where corporate mail servers often employ advanced filtering that goes beyond what consumer mailbox providers might use. The challenge lies in diagnosing what happens after your email service provider (ESP) reports an email as 'delivered'.
The distinction between an email being 'delivered' and 'received' in the recipient's inbox is crucial. An email can be successfully handed off to the receiving mail server, registering as 'delivered' on your end, but still fail to reach the intended recipient's mailbox due to various post-delivery processing rules. Let's explore the common culprits behind this issue and how to investigate them.
The gap between delivery and reception
One of the most common misunderstandings in email deliverability is the difference between an email being accepted by the receiving server and actually landing in the inbox. When your ESP reports an email as 'delivered,' it typically means the receiving mail server responded with a 250 OK SMTP reply. This indicates acceptance, not necessarily inbox placement.
After this initial acceptance, the recipient's mail server or security system can still filter, quarantine, or even silently drop the email. This is especially prevalent with corporate domains that have sophisticated spam and security filters in place, which might not issue a bounce message back to your sending system. These filters evaluate emails based on many factors beyond just basic authentication and opt-in status.
This 'black hole' effect can be frustrating because your data might show successful delivery, yet your customers report never seeing the emails. To uncover these hidden delivery issues, you need to go beyond standard bounce lists and dig deeper into post-delivery processing. This involves understanding the recipient's environment and the nuances of email flow after acceptance.
The silent drop: When 'delivered' doesn't mean 'inbox'
Many email service providers (ESPs) confirm email 'delivery' based on a 250 OK SMTP response from the recipient's mail server. However, this only signifies that the email was accepted, not that it reached the inbox or even the spam folder. Corporate mail filters, especially those used by organizations with strict security protocols, often quarantine or silently discard emails without sending a bounce notification back to the sender.
Quarantine: Emails are held for review by IT administrators. Customers won't see them unless released.
Silent filtering: Emails are dropped without a bounce or notification, making it seem like they vanished.
Why opted-in emails go missing
Even with explicit opt-in, several factors can prevent your marketing emails from reaching the inbox. Understanding these underlying issues is key to improving your deliverability.
Sender reputation
Your sender reputation, both for your IP and domain, is paramount. If your reputation is poor, mailbox providers are more likely to filter your emails, even for opted-in recipients. Factors like spam complaints, low engagement, high bounce rates, and being listed on a blocklist (or blacklist) can severely damage your standing. Regularly checking your reputation with tools like Google Postmaster Tools and other reputation services is crucial.
Authentication issues
Proper email authentication (SPF, DKIM, DMARC) is non-negotiable for deliverability. If your authentication records are misconfigured or failing, receiving servers will view your emails with suspicion. This can lead to them being rejected, sent to spam, or quarantined. Even if your ESP handles some of this, verifying your DMARC, SPF, and DKIM records is a critical step.
Content and engagement
The content of your emails and how recipients interact with them also play a significant role. Emails with characteristics often associated with spam, like excessive images, poor formatting, or certain keywords, can trigger filters. Moreover, if your opted-in subscribers rarely open or click your emails, or worse, mark them as spam, it signals to mailbox providers that your content isn't desired, negatively affecting future deliverability.
Recipient-side factors
For B2B clients, corporate firewalls and email security solutions add another layer of complexity. These systems can have their own internal blocklists (or blacklists), content filters, and quarantine procedures that operate independently of your ESP's delivery reports. It's possible the emails are being caught by these internal defenses. In some cases, recipients may also have individual spam settings that are overly aggressive, redirecting even desired emails.
Poor sender reputation: Mailbox providers deem your sending practices suspicious due to past behavior (e.g., spam complaints, low engagement). Check Google Postmaster Tools or Microsoft SNDS.
Authentication failures: Incorrect or missing SPF, DKIM, or DMARC records can lead to emails being flagged as unverified.
Throttling/deferrals: Mailbox providers temporarily delay emails, often due to high volume or minor reputation concerns. These can turn into silent drops.
Content & recipient-side issues
Spammy content: Trigger words, poor HTML, broken links, or suspicious attachments can flag your emails as spam, even to opted-in users.
Low engagement: If recipients rarely open, click, or reply, mailbox providers may start routing your emails to spam or blocking them altogether. This impacts future deliverability, especially to engaged customers.
Corporate filters: B2B clients often have robust internal spam filters and firewalls that can quarantine or block emails without notifying your ESP. Their IT team might need to whitelist your sending domain.
Incorrect segmentation: Even if opted-in, a customer might not meet specific criteria for a particular campaign segment, leading to them not receiving it.
To effectively troubleshoot these issues, you need to go beyond basic deliverability reports and perform a deeper analysis of your email program.
Steps to investigate missing emails
When you encounter opted-in customers not receiving your emails, a systematic investigation is required. Don't rely solely on basic ESP reports.
Dive into the data
Your ESP should provide detailed logs for each email, including the SMTP response codes. A 250 OK indicates acceptance, but it doesn't confirm inbox placement. Look for deferred messages, which are emails temporarily rejected by the recipient's server but which your ESP may retry sending. These deferrals can sometimes turn into silent drops or permanent rejections without a hard bounce.
For specific problematic domains, especially corporate ones, it's beneficial to analyze their Mail Exchange (MX) records to identify their mail server provider. If a large segment of your missing emails are going to Microsoft 365, Google Workspace, or a similar major provider, you can then focus your investigation on their specific filtering policies and tools.
Direct customer engagement
While relying solely on anecdotal reports from sales can be misleading, direct communication with affected customers is still a valuable step. Ask them to check their spam or junk folders, and any other filtered folders. Also, guide them on how to safelist your sending domain or email address. For B2B clients, encourage them to speak with their IT department to check their internal email security logs for any quarantined or blocked messages from your domain. Provide specific details like sending IP, domain, and message IDs to their IT team to help them locate the messages.
Understanding hidden filtering and quarantines
One common issue is when a mail filter quarantines an email. Your ESP might show the email as delivered because the receiving server accepted it with a 250 OK SMTP reply. However, the email then never reaches the user's mailbox because it's held in a quarantine by the recipient's mail filter.
This typically occurs more often with corporate email systems like Microsoft or Google Workspace instances, which have advanced filtering. While the message isn't 'dropped' entirely, it's not delivered to the user's inbox. IT teams managing these instances often have the ability to 'release' emails from quarantine once they locate them. This highlights the importance of collaboration with your B2B clients' IT departments.
Investigating email delivery beyond bounces
Understanding the journey of your emails is key to troubleshooting why opted-in customers aren't receiving them. Your ESP's reporting might show emails as 'delivered' based on the SMTP 250 OK response, meaning the receiving server accepted the email. However, this doesn't guarantee inbox placement. Especially with corporate domains, emails can be caught by internal spam filters and security systems, being quarantined or even silently dropped, without generating a bounce message.
To address this, consider these factors:
Quarantine checks: Advise recipients to check their email's quarantine folder or contact their IT department. Many corporate systems hold suspicious emails for review, allowing them to be released manually.
Email authentication: Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. Failures can lead to messages being flagged even if they pass initial delivery checks. A link to the SAP Community article explains this in more detail.
Views from the trenches
When opted-in customers aren't receiving your emails, it's often a sign that there are deeper deliverability challenges at play beyond simple bounces or suppression lists. Here are insights and tips from the community that can help.
Best practices
Always obtain explicit consent. Double opt-in is the gold standard for building a clean, engaged list.
Regularly monitor your domain and IP reputation using tools like Google Postmaster Tools and Microsoft SNDS.
Segment your audience based on engagement levels and tailor content to maintain interest and prevent flagging as spam.
Ensure SPF, DKIM, and DMARC are correctly configured and aligned for your sending domains. Check for errors.
Provide clear, easy-to-find unsubscribe links in every marketing email.
Common pitfalls
Assuming 'delivered' means 'inboxed.' Many ESPs only confirm acceptance by the receiving server, not actual inbox placement.
Ignoring anecdotal reports from sales or customers; these can be early indicators of hidden filtering issues.
Not segmenting lists, leading to sending irrelevant content to disengaged recipients who might mark emails as spam.
Failing to monitor blocklists (blacklists) and domain reputation regularly, missing crucial flags from mailbox providers.
Having misconfigured or missing SPF, DKIM, or DMARC records, which can cause emails to be filtered or rejected.
Expert tips
For B2B scenarios, always ask customers to check their spam/junk folders and, if necessary, contact their IT department to check quarantine logs.
Investigate deferral logs from your ESP; these can indicate temporary rejections that might later become silent drops.
Understand that corporate filters operate differently than consumer ones; they often employ stricter rules and custom blocklists (blacklists).
Maintain a consistent sending volume and pattern to avoid triggering volume-based filters at receiving mail servers.
Implement feedback loops with major mailbox providers to be notified when recipients mark your emails as spam, allowing you to remove them from your list.
Expert view
Expert from Email Geeks says to investigate the specific mailbox providers of the segment not receiving emails, especially if it's primarily Microsoft or Google Workspace.
2024-03-26 - Email Geeks
Expert view
Expert from Email Geeks says that a common gap is when emails are severely delayed or permanently deferred by the mailbox provider, as they might be in a reporting blind spot.
2024-03-26 - Email Geeks
Ensuring your emails reach their destination
Addressing the issue of opted-in customers not receiving marketing emails requires a comprehensive approach. It’s rarely a single problem, but rather a combination of factors related to sender reputation, authentication, content, and recipient-side filtering.
By understanding the distinction between email delivery and reception, diving deep into your ESP's detailed logs, monitoring your domain and IP reputation, ensuring proper authentication, and maintaining high-quality, engaging content, you can significantly improve your chances of reaching the inbox.
Remember, email deliverability is an ongoing process that requires continuous monitoring and adaptation. Proactive management of your email program will ensure that your valuable marketing messages consistently land where they belong: in the inboxes of your eagerly awaiting, opted-in customers.
Microsoft SNDS.
Microsoft or 
Google Workspace instances, which have advanced filtering. While the message isn't 'dropped' entirely, it's not delivered to the user's inbox. IT teams managing these instances often have the ability to 'release' emails from quarantine once they locate them. This highlights the importance of collaboration with your B2B clients' IT departments.
