Aside from storing emails in your inbox. ExtendSync can also help you sync emails when:
Sync email in compose mode
Once you are done drafting out the email, click the Send and Open with ExtendSync button. Once sent, ExtendSync lets you select the record in which you want to store the email.
Replying to an existing thread
When you have an email but want to start syncing the email after sending the reply from your inbox. Click reply and then after you compose your response, click the Send and Open with ExtendSync button.