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Can you send messages without attachments?
So you were doing this from within the Word app?
If it is opening your Apple Mail app you need to do this:
Set up your default email reader
Your default mail reader is what's used when you're browsing a web page and share it via email with a friend or colleague, or mail it to yourself for further reading. The Mac default is to use the Mail app, but you can change the default to Outlook.
Open Finder > Applications.
Select Mail > Preferences.
On the General tab, in Default email reader, select Microsoft Outlook.
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Change it to Mail then try it again, then change it back to Outlook
If the problem persists: you can reset the default email client that is saved by LaunchServices, it can become corrupted. You can reset the database with the following command in Terminal (copy-paste the whole thing then hit Return):
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user