To use a salesforce lightning default email option on the object page, we need to create a global action. For this, we have to go with the following steps:-
1) Go to Setup | Global Action
2) Create a new Send Email global action.
3) Go to the object manager and select the object in which you want to add this global action.
4) Go to Page Layout and choose the layout.
5) Choose Mobile & Lightning Actions.
5) Drag the Email action and drop it below into the Salesforce Mobile and Lightning Experience Actions.
6) Press save.
7) Go to the object page and open any record.
8) Open the Activity tab [Email will be present in this tab only].
9) click on the Email.
10) Now you can email.