Dear MC5577,
Good day! Thank you for posting to Microsoft Community. We are happy to assist you.
Based on your description, I understand that you are experiencing an issue with Outlook encrypting Excel attachments. I'll be happy to assist you with this.
Firstly, I would like to confirm if you are using the latest version of Outlook and Excel. If not, please update both applications to the latest version and check if the issue persists.
If you are using the latest version of both applications, then it is possible that your company has set up a policy to encrypt all attachments, including Excel files. I would recommend reaching out to your IT team again and asking them to check if there are any policies in place that could be causing this issue.
Additionally, you can try disabling the automatic encryption feature in Outlook by going to File > Options > Trust Center > Trust Center Settings > Email Security and unchecking the "Encrypt contents and attachments for outgoing messages" option.

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I hope this helps! Let me know if you have any further questions or concerns. Please understand that our initial response may not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.
Thanks for your precious time. Have a nice day.
Sincerely,
Sherry | Microsoft Community Moderator