I'm guessing that you want the emails to be encrypted before being saved to any disk? In that case, Switchboard isn't sufficient: it runs in front of email providers, and can't prevent them from saving an email to disk.

Switchboard could be used to fetch the email, encrypt it, and then replace the unencrypted email with the encrypted email on the server. However, replacing emails is invasive if the user doesn’t expect it, and other than the email provider's word you have no guarantee that the unencrypted email was actually deleted.