To send a scheduled 'one-on-one' Email under the 'Contact Details' popup, go to the 'Contacts' tab and select the 'Contact' to whom you would like to send the 'one-on-one' email, and select 'Email' on the popup as shown below.
From here, write your subject line, draft your email and you then have the ability to select a 'date and time' to send the email.
In doing so, a task will be created in your 'Sales Inbox' as shown below.
Click on the above 'Task' created under the 'Sales Inbox' and the 'Contact Details' popup will open up from where you will be able to make any required changes to the body of your email and 'Send' it out as highlighted below.