Derek Ashauer

TinyMCE has a built in feature to include a predefined list of links to select from. This plugin simply creates a list 
from your pages in WordPress. Create a link like usual in TinyMCE, but you now have a new drop down menu called “Link List” 
where all pages/links are already setup for you.
This currently only works for pages, not for posts.

AWS Easy Page Link

Using the External Link List feature of TinyMCE, this makes it easy to create a link to another page on your website.

kubiq

This plugin will replace raw textarea by TinyMCE WYSIWYG editor when “Use HTML content type” is checked. 
Works for both Mail and Mail (2) in Contact Form 7.

Contact Form 7 – email body TinyMCE editor

TinyMCE WYSIWYG editor when "Use HTML content type" is checked in Contact Form 7 Mail and Mail (2)

AWS Easy Page Link vs Contact Form 7 – email body TinyMCE editor