How to interpret Office 365 notifications for emails sent outside the organization?
Michael Ko
Co-founder & CEO, Suped
Published 23 May 2025
Updated 19 Aug 2025
7 min read
Receiving email notifications from Microsoft Office 365 regarding emails sent to or from outside your organization can sometimes be confusing. These alerts, ranging from simple warning banners to more detailed security notifications, are designed to enhance your organization's security posture and alert users to potential risks. However, understanding what each notification truly signifies is crucial for effective email management and security.
I'll help you decode these Office 365 notifications and offer insights into why they appear and what actions you might need to take. By the end, you'll have a clearer picture of how to interpret these messages, ensuring both security and smooth email flow.
What defines 'outside the organization'?
When Office 365 refers to outside the organization, it's not just about an email coming from an unfamiliar sender. It specifically refers to any email communication where the sender's or recipient's domain is not part of your configured Microsoft 365 tenant. Your organization, in this context, is defined by the domains you've added and verified within your Office 365 environment.
This distinction is critical because it forms the basis for many of Office 365's security features, including external email warnings and certain mail flow rules. Even if you regularly communicate with a partner company, their emails will be flagged as external simply because their domain isn't registered under your Office 365 tenant. This provides a baseline for users to exercise caution, as external emails inherently carry a higher risk of phishing or spoofing.
The outside the organization definition also plays a role in features like external email forwarding and message recall functionality. For instance, you typically cannot recall an email once it has been delivered to an external recipient because it has left your organization's immediate control.
Common external email notifications
Office 365 employs several types of notifications for emails exchanged with external entities. These are generally categorized into user-facing warnings and administrative security alerts.
User-facing warnings are often implemented via mail flow rules in Exchange Online. The most common is the [External] tag prepended to the subject line or a warning banner inserted into the email body. This visual cue prompts users to verify the sender's legitimacy before interacting with the email, significantly reducing susceptibility to phishing attacks.
Administrative security alerts, on the other hand, are generated by Office 365 security and compliance centers. These notifications typically inform administrators about suspicious email sending patterns detected or unusual activity involving outbound emails. Such alerts can signify a compromised account, an internal user sending spam, or even a legitimate third-party application sending emails incorrectly.
Other notifications include bounces for external recipients, where emails fail to deliver. These might indicate recipient server issues, invalid addresses, or even your domain being on a blocklist (or blacklist).
Deciphering the root causes of notifications
Interpreting these notifications effectively requires understanding the potential underlying causes. It's rarely a single issue, but rather a combination of factors.
One primary cause is often related to email authentication issues. If your SPF, DKIM, or DMARC records are misconfigured or missing, even legitimate emails sent from your domain might be viewed as suspicious by Office 365, leading to warnings or even delivery failures. While a notification might not explicitly say DMARC failure, underlying authentication problems can trigger broader suspicious sending pattern alerts.
Another common scenario involves using third-party email sending services. Many organizations use tools like CRMs, marketing automation platforms, or support ticketing systems that send emails on their behalf. If these services aren't correctly configured to align with your domain's authentication, Office 365 may flag their outbound messages as suspicious, even if they're legitimate business communications.
Finally, the most concerning reason for these notifications is a compromised user account within your organization. If an attacker gains access to an employee's mailbox, they might use it to send large volumes of spam or phishing emails to external recipients. Office 365's detection systems are designed to identify such anomalous outbound activity and alert administrators, helping to mitigate the damage and prevent your domain from being placed on an email blacklist (or blocklist).
Actionable steps to resolve notifications
Addressing these notifications systematically is key to maintaining good email deliverability and security. Here are the actionable steps I recommend taking:
Inspect headers: For any suspicious or unexpected notifications, always examine the email headers. This raw data can reveal the true origin of the email, the path it took, and any authentication results (SPF, DKIM, DMARC). It helps determine if the message is legitimate or if it's a spoofed attempt. You can usually find the option to view headers within your email client, often under File > Properties or Message options.
Strengthen authentication: Ensure your SPF, DKIM, and DMARC records are correctly set up and include all legitimate sending sources, including third-party providers. Regularly review your DMARC reports to identify any sources that are failing authentication.
Monitor for suspicious activity: Utilize Office 365's security and compliance center to monitor for unusual outbound sending patterns or other anomalous user behavior. Implement multi-factor authentication (MFA) to prevent account compromises.
User education: Train your users on the importance of external email warnings and how to identify phishing attempts. Encourage them to report suspicious emails using built-in Outlook features.
For managing user-facing warnings, such as the [External] tag, you can adjust mail flow rules in Exchange Online. However, it's generally advisable to keep these warnings enabled unless you have a very specific reason to disable them for certain trusted external senders, as they serve a vital security function. If your organization is experiencing issues with emails landing in spam folders, further investigation into Office 365's spam filtering may be necessary.
Properly interpreting these notifications and proactively addressing their root causes will significantly improve your email security posture and ensure reliable communication with external parties. This helps prevent your legitimate emails from being incorrectly flagged or quarantined by Office 365 for other organizations.
Views from the trenches
Best practices
Always verify the legitimacy of the notification before taking any action.
Regularly review your email authentication records (SPF, DKIM, DMARC) for completeness.
Ensure all third-party sending services are properly authenticated for your domain.
Educate users about external email warnings and how to identify phishing attempts.
Implement multi-factor authentication (MFA) for all user accounts.
Common pitfalls
Ignoring external email warning banners as mere nuisances.
Assuming DMARC failure is the only cause of
outside the organization
notifications.
Failing to review email headers to trace the true origin of suspicious emails.
Expert tips
Utilize Office 365's security and compliance center for detailed alert policies.
Access DMARC data to verify the origin and authentication status of emails.
Check if the message originated from a legitimate Microsoft IP address.
Be aware that third-party apps might send from their own infrastructure.
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Expert view
Expert from Email Geeks says to review the email headers to ensure the message legitimately originated from a Microsoft IP address if the notification comes from Office 365.
2019-07-29 - Email Geeks
Expert view
Expert from Email Geeks says that the
2019-07-29 - Email Geeks
Ensuring secure and reliable external email communication
Office 365 notifications for emails sent outside the organization are essential tools for maintaining email security and integrity. They serve as an early warning system against potential threats like phishing, spoofing, and compromised accounts, while also highlighting authentication issues.
By understanding the different types of notifications, their common causes, and taking proactive measures like strong email authentication and user education, you can effectively manage these alerts. This approach helps ensure that your organization's legitimate emails reach their intended recipients reliably and securely, while mitigating risks associated with external communication.
Microsoft Office 365 regarding emails sent to or from outside your organization can sometimes be confusing. These alerts, ranging from simple warning banners to more detailed security notifications, are designed to enhance your organization's security posture and alert users to potential risks. However, understanding what each notification truly signifies is crucial for effective email management and security.
