in the Wiki is stated:
https://help.mikrotik.com/docs/display/ROS/E-mailtls (no|yes|starttls; Default: no) Whether to use TLS encryption:
yes - sends STARTTLS and continue without TLS if a server responds that TLS is not available;
no - do not send STARTTLS;
starttls - sends STARTTLS and drops the session if TLS is not available on the server.
But in the ROS (v7.8 ) there is no "starttls" option, only "tls only". But it is not stated how this behaves?!