With the option “Edit Link” you can link draw.io shapes so that you can call your email client from the diagram. If you also want to add a subject, you have to embed the HTML option.
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Or learn more about diagramming with draw.io at https://drawio-app.com/learning. You can also take a look at our blog (https://drawio-app.com/blog) to always stay up-to-date on new draw.io features in Confluence.