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Why are Microsoft 365 emails vanishing when senders use specific symbols?
Urgent Warning: Silent Email Deletion in Microsoft 365
A critical issue has surfaced for Microsoft 365 users. Reports indicate that Exchange Online is silently deleting incoming emails if the sender’s display name contains specific special characters. This bug affects emails immediately. The sender receives no non-delivery report (NDR), and the recipient never sees the message.
The specific characters triggering this deletion are:
- Parentheses: (
- Ampersands: &
- Umlauts: ä
If a sender uses a name like “Müller & Sons (Logistics)”, the email may simply disappear. This behavior was first flagged by a reader, Holger S., on January 29, 2026. He noticed that valid emails were vanishing without a trace. This is not a spam folder issue; the emails are being completely removed from the pipeline.
Why This Is Happening
The root cause likely lies in how Exchange Online parses email headers. Email systems use specific protocols (like MIME) to read sender information. Special characters can disrupt this process if the system does not handle them correctly.
- Code Confusion: Characters like & and ( have special meanings in programming languages (HTML, XML, or script). If a security filter misinterprets these characters in a name field as malicious code, it might drop the email to protect the user.
- Encoding Failures: The character ä requires specific character encoding (like UTF-8). If an update to Microsoft 365 has temporarily broken the way the system reads these international characters, the filter might view the email header as “corrupt” and discard it.
This appears to be a bug in a recent update to the Exchange transport service. The system effectively “chokes” on the name string and fails safe by deleting the message.
Who Is at Risk?
This bug poses a significant risk to business operations. It does not matter if other characters are present; the presence of the trigger characters is enough.
- International Partners: Senders with German, Scandinavian, or other names using umlauts (ä, ö, ü) are prime targets.
- Corporate Senders: Companies often use & or parentheses in their official display names (e.g., “Smith & Wesson (HR)”).
- Automated Systems: Newsletters or transactional emails often format sender names with these symbols.
Because the sender gets no error message, they assume you received the email. You might be missing invoices, contracts, or urgent updates right now.
What You Should Do
If you administer a Microsoft 365 environment, you must act to verify this issue.
- Check Message Trace: Go to the Exchange Admin Center. Run a Message Trace for a sender known to use these characters. Look for status messages like “Failed” or “Drop”.
- Test Internal Flow: Send an email to yourself from an external account. Change your display name to include ( & ä. See if it arrives.
- Monitor Service Health: Check the Microsoft 365 Service Health Dashboard for advisories related to “Exchange Online” or “Mail Flow”.
Until Microsoft patches this error, advise your key contacts to simplify their display names if they suspect communication issues. Removing the special characters from the display name temporarily fixes the delivery path.