Simple Mail Transfer Protocol or SMTP has easily exploitable security loopholes. Email routing protocols were designed in a time when cryptographic technology was at a nascent stage (e.g., the de-facto protocol for email transfer, SMTP, is nearly 40 years old now), and therefore security was not an important consideration.
As a result, in most email systems encryption is still opportunistic, which implies that if the opposite connection does not support TLS, it gets rolled back to an unencrypted one delivering messages in plaintext.
