Some messages that come to the Inbox come in Plain Text format, which means that very few formatting features are available on replies and forwards. There is no ability to bold, italicize, add Quick Parts, etc. This can be changed to allow the current user more ability to format.
1. In a Plain Text formatted message, click on the “Format Text” tab. “Plain Text” will be selected and the formatting features will be grayed out.
2. Click on “HTML.” The formatting features will become active.
NOTE: A sign that a message may be in Plain Text format is when the reply/forward text does not change to the normal color that has been chosen for that (often blue).
These instructions can also be found in SharePoint > Software Users Group > Shared Documents > Outlook-Office 365 Folder > Changing a Reply/Forward to HTML Format.