SMTP is the protocol that is used for basically all email, and it does not provide encryption. There are versions of the protocol that DO, however they cannot be used in isolation. Emails hop from source to destination via a bunch of SMTP relay servers, and since nearly all SMTP servers support the legacy protocol and do not support key exchange, encrypted SMTP traffic will bounce.
When you use gmail, your connection to Google is secure. But if the recipient of your message is not @gmail.com, the message leaves Google's servers in plain text over SMTP. If the recipient is @gmail.com, the message stays inside the Google network.
The point is, all of our emails traverse the internet in plain text unless we use custom solutions (eg. PGP at both endpoints) or send emails within a network (eg. Gmail to Gmail).