Gmail supports secure email transport via STARTTLS, which is considered a very basic security measure. It has many weak spots and above all, STARTTLS is an opportunistic protocol.
Why? Let’s get a little technical here. Even though nowadays enabling TLS should be one click away, this doesn’t solve the problem, as you have a dependency on your recipient’s server settings. To create a secure connection, both the sender and recipient must use TLS. When a secure connection can't be created, Gmail sends via non-secure connections - or not at all. Zivver for Gmail ensures messages are delivered securely.