User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Mail Format > Internet Formatting
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail
This policy setting controls how Outlook sends Rich Text Format (RTF) messages to Internet recipients. If you enable this policy setting, you may choose from the following for handling RTF messages addressed to recipients on the Internet: * Convert to Plain Text format - Outlook converts the message to plain text format in the default character set. Any message formatting will be lost. If you disable or do not configure this policy setting, Outlook automatically converts RTF formatted messages that are sent over the Internet to HTML format, so that the message formatting is maintained and attachments are received.