Why are emails with email addresses in the subject line being blocked by Office 365?
Michael Ko
Co-founder & CEO, Suped
Published 23 Jul 2025
Updated 19 Aug 2025
8 min read
Emails with email addresses in the subject line are a common cause of deliverability issues, particularly with Microsoft Office 365. Many senders experience their legitimate emails being unexpectedly blocked or quarantined, leading to communication breakdowns and frustration. Understanding why Office 365 implements these stringent filters is the first step toward resolving the problem.
These types of subject lines are often flagged due to their association with phishing, spoofing, and spam campaigns. Cybercriminals frequently embed recipient or sender email addresses in the subject to trick users into thinking the email is legitimate or urgent. Office 365's robust security mechanisms, including Exchange Online Protection (EOP) and Microsoft Defender for Office 365, are designed to detect and prevent such deceptive practices.
The challenge for legitimate senders is that their emails, despite being benign, can inadvertently trigger these security filters if they include email addresses in the subject line. This can lead to your emails being sent to the junk folder or even outright blocked, without a clear bounce message indicating the specific reason. The lack of a bounce can make troubleshooting particularly difficult, as the mail server accepts the message but never delivers it to the inbox.
Why Office 365 flags subject lines with email addresses
Office 365 employs a multi-layered approach to email security, and subject line content is a significant factor in its filtering decisions. When an email address is detected in the subject, it raises a red flag for the system, often indicating a potential phishing attempt or email impersonation. These are aggressive measures, but necessary to protect users from the constant barrage of threats.
Anti-phishing and anti-spoofing measures
One of the primary reasons is to combat phishing and spoofing. Phishing attacks often use subject lines that mimic legitimate communication, sometimes by including an email address (e.g., 'Payment overdue from [email protected]'). This tactic aims to build false trust or create urgency. By blocking these, Office 365 helps to shield users from fraudulent emails.
Spam filtering intelligence
The spam filter, including technologies like Microsoft's SmartScreen, is constantly learning and adapting to new spam techniques. If a significant volume of spam or malicious emails are observed using email addresses in the subject line, this pattern becomes a high-confidence indicator of unwanted mail. This can lead to a general blocklist (or blacklist) rule being applied to such content.
Default and custom mail flow rules
Office 365 administrators also have the ability to set up mail flow rules (transport rules) that specifically target keywords, patterns, or even the presence of email addresses in the subject line. These custom rules, often implemented for data loss prevention (DLP) or internal security policies, can override default spam filters and lead to messages being blocked or quarantined.
Troubleshooting when emails are accepted but not delivered
When your emails are getting blocked without a clear bounce message, it often means that Office 365 has accepted the email initially, but then it was either silently dropped, sent to the junk folder, or quarantined at the recipient's end. This silent failure makes troubleshooting more complex than a typical hard bounce with an error code.
Checking message trace
The first step in diagnosing this issue is to perform a message trace within the Microsoft 365 admin center. This tool provides detailed information about an email's journey, including whether it was delivered, quarantined, or rejected, and the specific reason for any non-delivery. This can help identify if a specific filter, policy, or anti-spam rule is the culprit.
Understanding quarantine and junk folders
Even if an email isn't outright blocked, it may be sent to the recipient's junk email folder or quarantined by Office 365. Users or administrators can usually access and review quarantined messages. If emails consistently end up here, it confirms that Office 365's filters are classifying them as suspicious, likely due to the subject line content.
Many deliverability issues with Microsoft domains often stem from a combination of content analysis and sender reputation. If your domain or IP has a low reputation, these content-based flags become even more impactful, increasing the likelihood of filtering. You can understand more about email domain reputation for better insights.
Solutions and best practices
The most straightforward solution is to avoid including email addresses in your subject lines. While this might seem obvious, sometimes business processes or automated systems unintentionally add this information. Re-evaluating your email templates and workflows is crucial.
Reviewing and modifying mail flow rules
If you are an administrator for your Office 365 environment, check the existing mail flow rules (also known as transport rules) in the Exchange admin center. Look for rules that might be configured to quarantine or reject messages based on subject line content, especially those containing special characters or email address patterns. You may need to create exceptions or modify these rules to allow legitimate emails through. You can learn more about why emails get quarantined and how to fix them.
Ensuring proper email authentication
While not directly related to subject lines, strong email authentication (SPF, DKIM, and DMARC) significantly boosts your sender reputation. A robust sender reputation can sometimes allow borderline emails to pass through filters that would otherwise block them. Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. This is fundamental to good email deliverability in general.
Here's an example of a mail flow rule that could be blocking emails based on subject content:
Example Mail Flow Rule in Office 365text
Rule: Block emails with email addresses in subject
Apply this rule if:
The subject or body includes any of these words or phrases: '[email protected]', 'user@domain.com', '.com', '.net'
Do the following:
Block the message (reject the message and include the explanation)
Explanation: 'Emails with addresses in subject lines are not allowed for security reasons.'
or
Redirect the message to: quarantine
If you suspect an issue with your current blocklist status, consider using a blocklist checker to see if your sending IP or domain is listed. While subject line content is distinct from blocklisting, a poor reputation can amplify other filtering triggers.
Advanced troubleshooting and Microsoft support
Sometimes, the issue isn't a custom rule, but rather Microsoft's internal SmartScreen filters or other advanced threat protection mechanisms. These systems are constantly evolving and may implement new blocklist (or blacklist) rules based on emerging threats.
Engaging Microsoft support
If you've exhausted other troubleshooting steps, opening a support ticket with Microsoft (Office 365) is often necessary. Provide them with specific message IDs from your sent emails, recipient details, and the exact subject lines that are causing issues. Be prepared to explain your use case and why the email address is included in the subject. You may need to escalate the ticket if initial responses don't resolve the issue, especially if emails are being accepted but never delivered.
They might be able to whitelist your specific sending pattern or provide insights into why their filters are flagging your messages. Be aware that Microsoft's default posture is highly protective against anything that resembles phishing or spam, so convincing them to make an exception might require a clear justification for your email practice.
Understanding why Microsoft blocks automated emails and how their policies work is key to effective communication with their support teams and for preventing future blockages.
Navigating Office 365's strict filtering
Office 365's filters are designed to be proactive against evolving threats. While this can sometimes catch legitimate emails, especially those with unusual subject line patterns like embedded email addresses, it's ultimately for the protection of its users. The best approach is to align your email practices with these security expectations.
By understanding the underlying reasons for blocking and implementing best practices such as avoiding problematic subject line content and ensuring robust email authentication, you can significantly improve your email deliverability to Office 365 recipients. This proactive stance helps maintain a healthy sender reputation and ensures your messages reach their intended inboxes without being flagged by a blocklist (or blacklist).
Views from the trenches
Best practices
Avoid placing full email addresses or suspicious character patterns within your subject lines to prevent triggering spam filters.
Regularly monitor your email deliverability and look for patterns in blocked or quarantined messages.
Configure strong SPF, DKIM, and DMARC records to authenticate your emails and build sender trust.
Common pitfalls
Ignoring silent email blocking where messages are accepted but never delivered to the inbox.
Failing to review or create exceptions for mail flow rules that may be inadvertently blocking legitimate emails.
Assuming a lack of bounce messages means the email was successfully delivered without checking.
Expert tips
Always keep your email authentication records up to date as a foundational element of good deliverability.
Educate your team on email best practices to avoid common pitfalls that lead to messages being blocked.
Leverage DMARC reports to gain visibility into how your emails are being authenticated and handled by recipients.
Expert view
Expert from Email Geeks says that mail being accepted then deleted is a Microsoft SmartScreen specialty, and it's essential to open a ticket with Microsoft support and keep asking for escalation until someone can help, describing the issue as mail accepted but never delivered.
2024-09-05 - Email Geeks
Expert view
Expert from Email Geeks says that emails in subject lines are generally a very bad idea for deliverability.
Microsoft 365 admin center. This tool provides detailed information about an email's journey, including whether it was delivered, quarantined, or rejected, and the specific reason for any non-delivery. This can help identify if a specific filter, policy, or anti-spam rule is the culprit.
