Hi Carina G,
Thank you for posting your question in the Microsoft Q&A forum.
Based on your description, here are some suggestions for you:
- Please check if this issue also occurs when forward email from OWA.
- Do you use New Outlook for Outlook for Microsoft 365.
Here is a similar issue for New Outlook: E-mail addresses disappear in the New Outlook when I reply or forward - Microsoft Community
If you are using Outlook for Microsoft 365 or other versions, you can try to upgrade to the latest version and check if issue persists.
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