Why can't I open Excel attachments in the new Outlook?

The new Outlook cannot open Excel attachments if their filenames contain non-ASCII characters due to an encoding issue Microsoft is addressing.

When will Microsoft fix the Outlook Excel attachment problem?

Microsoft is developing a fix for the Outlook Excel attachment issue, but a release date has not yet been provided.

Has the NHS reported issues with the new Outlook?

Yes, major international organizations like the UK's National Health Service (NHS) have reported problems with users being unable to open Excel attachments in the new Outlook.

Home / Technology / New Outlook Bug Locks Out Excel Files

New Outlook Bug Locks Out Excel Files

3 Dec, 2025

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Summary

Outlook users cannot open Excel attachments with non-ASCII characters.
Microsoft confirmed the issue and is developing a fix.
The problem has affected users for at least two weeks.

The new Outlook email client is experiencing a significant bug that prevents users from opening Excel attachments containing non-ASCII characters in their file names. This issue, which has persisted for at least two weeks as of December 3rd, 2025, has been confirmed by Microsoft and is identified by service alert ID EX1189359.

Microsoft has stated that the problem arises from a missing encoding in the requests used to open these files. Users attempting to access affected attachments receive a message suggesting they try again later. Major organizations, including the UK's National Health Service, have reported experiencing this frustrating problem.

While Microsoft has confirmed that a fix is being developed and validated, a timeline for its deployment has not yet been announced. This ongoing issue highlights potential vulnerabilities in the new Outlook client's handling of file attachments with special characters, impacting user productivity.