Puts the connection to the SMTP server into TLS mode.
If there has been no previous EHLO or HELO command this session, this
method tries ESMTP EHLO first.
If the server supports TLS, this will encrypt the rest of the SMTP
session. If you provide the context parameter,
the identity of the SMTP server and client can be checked. This,
however, depends on whether the socket module really checks the
certificates.
This method may raise the following exceptions:
SMTPHeloError The server didn't reply properly to
the helo greeting.