Plaintext email is message content that is not encrypted. Anyone able to inspect the traffic or access an unprotected system handling the message may be able to read it. Transport encryption, such as TLS, can protect a connection between mail servers, but it does not necessarily encrypt the message end to end; protection may end at a server or network boundary.
This matters when email crosses untrusted networks, including traffic leaving a Tor exit node: Tor can obscure a sender’s route, but it does not encrypt message content on its own. Attackers who can observe an unprotected connection may read messages or credentials. Use encrypted connections to mail services and, for sensitive content, end-to-end message encryption; do not treat routing privacy as content confidentiality.