Users may report that the Standard Email Quick Action is not visible in the Salesforce Case Feed.
If the user's email is set to send through Office 365 or Gmail, ensure the user has the "Send Email through External Email Service" permission set.
This behavior is typically caused by the org's email access level being set to System Email Only. Additionally, Email-to-Case must be enabled in the org before the Email quick action becomes available on the Case Feed.
Note: After you enable Email-to-Case, you cannot disable it.
If the Email action is still unavailable after the above steps, check the following under Support Settings:
Guidelines for Configuring Deliverability Settings for Emails Sent from Salesforce
Troubleshooting Email Actions
Email publisher action is not visible on Case after it is added to the page layout
Configure the Case Feed Publisher with the Enhanced Page Layout Editor
Salesforce Trailblazer Community - Email tab in Chatter Publisher
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