The visibility of buttons and actions on a Lightning record page depends on both the page layout configuration and the type of button.

1. When a Case page layout is feed-based (includes the Case Feed component), standard actions like 'Email' are consolidated into the Chatter publisher instead of appearing as individual buttons in the highlights panel.

2. Custom buttons that rely on JavaScript for their functionality are not supported in Lightning Experience. If the "email button" is a custom JavaScript button, it will not render on the Lightning record page, even if it is correctly added to the page layout.

C. The case page layout also contains custom buttons.

The presence of other custom buttons does not inherently prevent a correctly configured email action or button from being displayed.

D. The page layout excludes the case feed component.

Excluding the Case Feed component would result in a standard page layout, where the email button is more likely to appear in the highlights panel if configured correctly.

1. Salesforce Help, "Actions in Lightning Experience": Under the section "Where Actions Appear," the documentation states, "For cases, leads, accounts, contacts, and opportunities, record-specific actions appear in the Chatter publisher." This confirms that on feed-based layouts, the Email action moves to the publisher.

2. Salesforce Help, "Create Custom Buttons and Links": In the notes for creating custom buttons, it is explicitly stated, "In Lightning Experience, custom buttons that use a JavaScript content source are not supported." This directly supports why a JavaScript-based button would not be visible.

3. Salesforce Help, "Set Up Case Feed": This document details the user interface for feed-based page layouts, showing actions like Email, Post, and Log a Call within the feed publisher, distinct from the standard button area of a non-feed-based layout.