How to troubleshoot email deliverability issues to specific business domains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 May 2025
Updated 19 Aug 2025
8 min read
Email deliverability issues can be incredibly frustrating, especially when emails seem to vanish into a black hole after being sent to specific business domains. There are no bounce messages, no opens, and no clicks, leaving you completely in the dark. This scenario often points towards a recipient-side filtering issue, possibly involving a common email security vendor used by these organizations.
When your overall deliverability is strong, with low bounce rates and good data hygiene, these isolated incidents highlight the need for targeted troubleshooting. It's about peeling back the layers to understand why particular domains are rejecting or quarantining your messages. Let's explore how to diagnose and resolve these elusive email deliverability problems, starting with identifying the underlying cause.
Initial diagnosis and domain investigation
When emails aren't reaching specific business domains, the first step is to investigate the recipient's mail infrastructure. This involves looking at their MX (Mail Exchanger) records, which indicate the mail servers responsible for receiving emails for that domain. If multiple affected domains share the same MX records, it strongly suggests a common email security gateway or service is in play. You can query these records using command-line tools.
Checking MX Recordsbash
dig MX example.com
nslookup -type=MX example.com
Once you have the MX records, you can further investigate by attempting a Telnet connection to port 25 of the mail server identified in the MX record. This allows you to interact directly with the receiving mail server and observe its EHLO (Extended Hello) response. The EHLO response often reveals the name of the mail server software or security appliance, which can confirm if a specific vendor, like Proofpoint, is involved.
Telnet for EHLO responsebash
telnet mail.example.com 25
EHLO yourdomain.com
In cases where a common security vendor is identified, it's crucial to understand their typical filtering behaviors. Many enterprise-level solutions have sophisticated spam and threat detection mechanisms that might quarantine emails without generating a bounce message. This means the email was accepted by the recipient's server but then held for review or blocked internally, leading to the "black hole" effect you observed. Understanding how to diagnose email deliverability issues is key.
Additionally, check for any visible signs that these business domains have updated their email security policies. Some organizations might have very strict configurations, especially if they've recently experienced a phishing attack or a surge in unwanted mail. Reviewing your own email logs to see the SMTP response code for these specific domains can also provide clues, even if it's just a 250 OK, which means the message was accepted but could still be quarantined later.
Authentication and reputation fundamentals
Your sender reputation is paramount. Even if you have good overall deliverability, a specific domain or IP address within your sending infrastructure might be flagged by a particular security vendor. It's important to ensure your email authentication records – SPF, DKIM, and DMARC – are correctly configured and aligned. Misconfigurations can lead to emails being rejected or quarantined, especially by stricter recipient servers.
You can use tools to confirm that your domain isn't listed on any major public blocklists (blacklists). While being listed on a public blocklist usually causes bounces, private blocklists or internal scoring mechanisms of enterprise security vendors might silently filter your mail. Regularly checking your domain's health and ensuring proper authentication helps build and maintain a strong reputation, which is critical for B2B deliverability.
SPF: Ensure your SPF record correctly lists all authorized sending IP addresses and domains. Incorrect or missing SPF records can lead to emails being marked as suspicious, especially for intermittent email delivery failures.
DKIM: Verify your DKIM signatures are correctly applied and validated. A valid DKIM signature helps prove the email hasn't been tampered with in transit.
DMARC: Implement a DMARC policy to monitor and enforce authentication. This provides insights into how your emails are being handled by recipient servers and is crucial for troubleshooting DMARC failures and improving trust.
Blacklist (Blocklist) Status: Regularly check if your sending IP or domain is listed on any email blocklists. While less likely to cause silent black-holing for legitimate B2B mail, it can still contribute to reputation issues.
Even with perfect authentication, content can sometimes trigger filters. Enterprise spam filters are highly sophisticated and analyze email content for characteristics typically associated with spam or malicious mail. This includes suspicious links, certain keywords, or unusual formatting. If your emails to these specific business domains consistently disappear, review your content for any patterns that might be triggering these filters.
Addressing security vendor specific challenges
Once you've identified a common security vendor, like Proofpoint or Microsoft 365, as the common denominator, the next step is to understand their specific filtering mechanisms. Enterprise solutions often employ robust quarantine features that hold suspicious emails for review by recipient IT teams, without sending a non-delivery report (NDR) back to the sender. This is a common reason for emails simply disappearing.
Common problems with enterprise filters
Quarantine: Emails are caught by spam filters and held in a quarantine folder, awaiting review by the recipient or their IT department. No bounce is generated.
Aggressive Filtering: High spam scores due to content, links, or perceived sender reputation can lead to rejection without explicit notification.
Greylisting: A temporary rejection that requires the sending server to retry. If not configured correctly, this can lead to delays or non-delivery.
The most effective way to resolve this is through direct communication. Try to reach out to the postmaster or IT department of the affected business domains. Explain the situation and ask them to check their email logs or quarantine for messages from your sending domain. Providing specific examples of emails sent (timestamps, recipient addresses) can be very helpful.
Many email security vendors, like Proofpoint, offer dedicated postmaster contacts for deliverability issues. Reaching out to their postmaster team can often provide insights into why your emails are being filtered and what steps you can take to mitigate the issue. They might inform you of reputation issues, content flags, or specific policies affecting your mail, which can differ from general Microsoft 365 email delivery issues. Remember that building a relationship with these postmaster teams can be beneficial for long-term deliverability.
Once you establish contact, you might need to request that your sending domain or IP address be whitelisted (allowed) by their security system. Be prepared to provide evidence of your legitimate sending practices, such as how you acquire contacts, your unsubscribe process, and your email authentication setup.
Content and list hygiene considerations
Even with technical configurations in place, the content of your emails can influence deliverability to specific business domains. Enterprise filters are sensitive to certain phrases, attachment types, or even specific link patterns that might be deemed suspicious. If a group of domains in the same industry is affected, their security policies might be tailored to detect threats common to that sector.
Maintaining a clean and engaged recipient list is also fundamental. Even for B2B lists gathered via forms and webinars, inactive or invalid addresses can lead to issues over time. While the client's overall bounce rate is low, it's worth considering the possibility of spam traps or abandoned domains among the affected recipients. Regularly validate your lists and remove unengaged contacts to prevent future deliverability problems. For a more comprehensive approach, consult a guide to improve email deliverability.
Views from the trenches
Best practices
Always begin troubleshooting by checking the MX records of the affected domains to identify their mail servers and security vendors.
Use Telnet to port 25 and analyze the EHLO response to confirm the specific email security gateway in use.
If a common security vendor is found, contact their postmaster or support team directly to inquire about your sending reputation and potential filtering.
Ensure your SPF, DKIM, and DMARC records are impeccably configured for optimal email authentication.
Common pitfalls
Assuming a lack of bounce messages means the email was successfully delivered, rather than quarantined or silently dropped.
Neglecting to investigate the recipient's email infrastructure and relying solely on sender-side checks.
Failing to communicate directly with the IT departments or postmasters of the specific business domains experiencing issues.
Ignoring the impact of email content and links, which can trigger sophisticated enterprise spam filters.
Expert tips
Verify your sending IP and domain are not on any blocklists (blacklists), even if you're not seeing bounces. Some private blocklists operate silently.
Be prepared to provide detailed sending logs and authentication status when contacting recipient IT teams or security vendors.
Consider segmenting your B2B sends if a specific industry or group of domains continues to show deliverability issues, allowing for tailored approaches.
Use Google Postmaster Tools, if applicable, to monitor your domain's reputation with Google, which can indirectly influence other large recipients.
Expert view
Expert from Email Geeks says to always check the MX records first to identify the recipient's mail infrastructure. This is the starting point for any specific domain deliverability issue.
2020-02-12 - Email Geeks
Expert view
Expert from Email Geeks says after checking MX records, try a Telnet connection to port 25 on the identified MX servers to see the EHLO response. This can reveal the specific security vendor they are using.
2020-02-12 - Email Geeks
Restoring your inbox reach
Troubleshooting email deliverability issues to specific business domains requires a systematic approach, combining technical analysis with direct communication. It's often not about broad deliverability problems but rather targeted filtering by sophisticated enterprise email security solutions.
By diligently investigating MX records and EHLO responses, ensuring your authentication is solid, and proactively engaging with recipient IT teams or security vendors, you can uncover the root cause of these mysterious email disappearances and restore your deliverability to these critical contacts.
Proofpoint or 
Microsoft 365, as the common denominator, the next step is to understand their specific filtering mechanisms. Enterprise solutions often employ robust quarantine features that hold suspicious emails for review by recipient IT teams, without sending a non-delivery report (NDR) back to the sender. This is a common reason for emails simply disappearing.
