Even though "New Email" button has been added to the Lightning button bar on the Account page layout, the Button does not show up in Salesfoce Lightning page layout for the Account page layout. Is there a bug with this in Lightning?
[SalesForce] “New Email” button does not display in Lightning
Related Solutions
To enable the 'Email Reservation' button, a PDF must be created (by clicking 'Create PDF' and saving the record. Once the new pdf record has been added to the 'Quote PDF's' related list, then the 'Email Reservation' button will become active.
Go to setup and search for Deliverability. Set Access Level to All Email
If it is set to System email only, or No access, it will hide the email actions on the pagelayout.
Best Answer
Is your org set to All Email, for the email action to be visible :
Setup -> Email -> Deliverability