In order to use email securely to communicate you have to pay attention to the following points:
Choose an email provider from that is well vetted do a search around dread to find one. One that allows Tor users and is known for not being very responsive to government requests.
The email provider should be completely usable without having to enable Javascript.
Always use PGP to encrypt the emails you send and make sure that your communication partner does the same too.
Never give away information in the subject field. Although the content of your message is encrypted with PGP you can still give away information with the unencrypted subject field. For example do not use “about the $4k drug deal we made” as a subject but rather something like “subject”.