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Navigate to the Universal Plugin Manager and select 'E-Mail this Page'
Select 'Configure'
Click on the magnifying glass icon to select the Confluence groups that are permitted to send e-mails (alternative: you can enter the names and split them with a comma)
Now click on 'Save'
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E-Mail Stylesheet is a feature that needs some CSS knowledge to use. Some content on the page needs to have a different style while sent as an email. We can define a custom style for them using this option.
Here are the steps to create a custom style:
Open the HTML inspector of the browser on the page
Inspect the content to apply style
Define a "CSS selector" for it (using id, class, attribute, or tag name).
Test this selector by adding it to the CSS section of the inspector
Add new CSS into the selector and check the result
copy the selector and CSS into the app configuration and save it
Here you can see these steps for a confluence page: