Configure the Email Action in the Activity Composer in Lightning Experience | Salesforce
Is the Email action missing from the activity composer on opportunities or other objects in your Salesforce org? Check your layout and permissions settings.
|Available in Lightning Experience in: Professional, Enterprise, Performance, Unlimited, and Developer Editions|
If the Email action is missing from the activity composer, open the page layout assigned to the record type for the opportunity or other object.
Add the Send an Email action (not the Send Email action) to the Salesforce1 and Lightning Experience Actions section of the page layout.
Make sure that your users have the Send Email (not Send an Email) permission.