Saying that an email is “encrypted” is incomplete without saying where.
Transport encryption
SMTP TLS protects a connection between two systems while the message is moving across that hop. The next system normally receives the message in a form it can process.
Message-level protection
Technologies such as S/MIME can encrypt or sign message content itself. This can provide protection that travels with the message, but it introduces certificate, key-distribution, client-support and recovery considerations.
Storage is another layer
Encryption at rest protects stored systems or media under a different threat model. It does not replace secure transport or message-level controls.