If you begin the email thread in Workable then we will automatically import any new replies. This process can take up to 20 minutes. If the email thread started in your Inbox you will need to trigger the import manually. Locate the candidate in Workable and at the bottom of the candidate's Timeline you will see an option to import emails between you and the candidate. Click the link to import. After doing it once, you don't have to do it again for this thread.
If your account does not have an email integration enabled, then you should begin all email correspondence through Workable.
After starting the email thread in Workable, you will be CC'd on the candidate's reply in your regular email. From that point you can reply via regular email and the communication will sync to Workable automatically. Email communication started outside of Workable is not able to sync back to Workable.