SMTPS is also called SMTP over SSL. It runs on a different port than the regular email port (465 by default). To connect with SMTPS, the client needs to start the TLS handshake directly at the very beginning.

STARTTLS is a command that runs on a regular email service port, 25 by default. If the server supports STARTTLS, this command shows up in the welcome banner and the client runs it to establish a TLS session to protect all subsequent communication. If the server does not support this feature, it will not advertise the STARTTLS command and the client will use clear text communication. The STARTTLS command is more flexible than SMTPS.

Although this document mainly covers STARTTLS, most is applicable to SMTPS.