Fix Microsoft 365 Emails Appear Sent but Are Not Delivered
Question details
Automated emails sent from a Microsoft 365 mailbox appear in the sender's Sent Items folder but are not received by internal recipients.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Sending automated internal emails with attachments using a script, which suddenly stopped delivering after exceeding potential threshold limits.
- Observed behavior
- Messages are marked as sent in the originating mailbox but do not appear in the recipient's Inbox, Junk, or Deleted Items folders due to silent drops, throttling, or quarantine rules.
Ensure you have Exchange Administrator permissions in Microsoft 365, as troubleshooting mail flow rules, message traces, and quarantine policies requires tenant-level administrative access.
Run a Message Trace in the Exchange Admin Center
Running a message trace is the most reliable method to determine exactly where an internal message was delayed, dropped, or quarantined.
A message trace follows email messages as they travel through your Exchange Online organization. It will reveal if a transport rule or spam filter intercepted the automated script emails before they reached the recipient's mailbox.
Log in to the Microsoft 365 admin center and navigate to the Exchange Admin Center (EAC).
In the left-hand menu, go to 'Mail flow' and select 'Message trace'.
Click 'Start a trace', enter the sender's email address (the automated script account) and the recipient's email address, and adjust the time range to when the emails failed.
Review the trace results. Look for statuses like 'Failed', 'Quarantined', or 'Filtered as Spam'. Click on the specific message to view detailed delivery events and error codes.
Check Restricted Entities and Outbound Spam Policies
Microsoft 365 automatically blocks accounts that send bulk automated emails if they exceed system sending limits to prevent spam.
Review Defender Quarantine and Mail Flow Rules
Missing messages might be held in the organizational quarantine due to strict attachment policies or internal transport rules.
Boost Your Productivity with WPS Office
While resolving Microsoft 365 administrative and email delivery issues, consider streamlining your daily document tasks with WPS Office. It is a lightweight, completely free alternative that handles all your word processing, spreadsheet, and presentation needs with native compatibility.
- 1. Download WPS Office: Visit the official WPS Office website and click the free download button for your operating system.
- 2. Install the Software: Run the downloaded installer and follow the simple on-screen instructions to set up the office suite.
- 3. Open Existing Documents: Launch WPS Office and directly open your existing Microsoft Office files with perfect formatting preservation.
Frequently Asked Questions
Why do automated script emails suddenly stop delivering after working fine?
Microsoft 365 continuously monitors sending patterns. If a script sends a sudden high volume of emails or attachments, it can trigger automated outbound spam protection, causing the system to temporarily throttle or restrict the mailbox from sending further messages.
Will the sender receive a bounce-back message if they are restricted?
Normally, a Non-Delivery Report (NDR) is generated. However, if messages are silently dropped by internal transport rules or sent directly to the tenant quarantine, no bounce-back may be generated, making it look like the email was sent successfully.
Does the recipient's junk folder catch these missing internal emails?
It is possible, but if the emails are blocked at the tenant level (quarantined by Defender or dropped by transport rules), they will never reach the recipient's mailbox at all. This means they will not appear in the Inbox, Junk, or Deleted Items folders.
Can I bypass Microsoft 365 spam filtering for internal automated emails?
Yes, Exchange administrators can set up mail flow rules (transport rules) in the Exchange Admin Center to set the Spam Confidence Level (SCL) to -1 for specific internal sender IP addresses or domains, effectively bypassing spam filtering.
