What is STARTTLS?

STARTTLS (or Oportunistic Transport Layer Security) refers to extensions in plaintext communication protocols that provide a way to convert a plaintext connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication.

Several protocols use a command called “STARTTLS” for this purpose. It is a form of opportunistic encryption that is primarily intended as a countermeasure to passive surveillance.

So STARTTLS responds to plaintext requests and offers an encrypted transmission. Or it responds to an encrypted request with a corresponding continuation of the encryption.

STARTTLS therefore forces encrypted transmissions, even on mail server receive ports set up for plaintext.

See also:
Wikipedia: STARTTLS