insert new Task(
    Subject = 'Subject: Test email....', 
    WhoId = myContact.Id, 
    WhatId = myOpportunity.Id,
    TaskSubtype = 'Email',
    ActivityDate = Date.today(),
    Description =  'emailtest1',
    Status = 'Completed'
);

Check out more details here: How to create email / SMS activity using Apex

and How to use an Apex Email Service to store emails on a record EXACTLY as Salesforce does