User Configuration > Administrative Templates > Microsoft Word 2016 > Word Options > Advanced > E-mail Options
User
HKCU\software\policies\microsoft\office\16.0\common\mailsettings
This policy setting allows you to disable compression on inlined images in Outlook. If you enable this policy setting, Outlook won't compress inlined images in the message. If you disable or don't configure this policy setting, inlined images would be compressed. Note: This policy setting only applies to subscription versions of Office, such as Microsoft 365 Apps for enterprise.