class Net::IMAP

Cached #capabilities will be cleared when this method completes.
Server capabilities may change after #starttls, #login, and #authenticate.

+STARTTLS+ capability.
Clients should not call #starttls unless the server advertises the
===== Capability

Related: Net::IMAP.new, #login, #authenticate

with the response (e.g. CAPABILITY) is insecure and cannot be trusted.
TLS starts immediately _after_ that response. Any response code sent
TaggedResponse to STARTTLS is sent clear-text, _before_ TLS negotiation.
Any #response_handlers added before STARTTLS should be aware that the
>>>
*Note:*

secured.
both successful. Any error indicates that the connection has not been
This method returns after TLS negotiation and hostname verification are

See DeprecatedClientOptions#starttls for deprecated arguments.

SSLContext[https://docs.ruby-lang.org/en/master/OpenSSL/SSL/SSLContext.html].
the keys are names of attribute assignment methods on
{OpenSSL::SSL::SSLContext#set_params}[https://docs.ruby-lang.org/en/master/OpenSSL/SSL/SSLContext.html#method-i-set_params];
Any +options+ are forwarded directly to

to start a TLS session.
Sends a {STARTTLS command [IMAP4rev1 §6.2.1]}[https://www.rfc-editor.org/rfc/rfc3501#section-6.2.1]

def starttls(**options)
  @ssl_ctx_params, @ssl_ctx = build_ssl_ctx(options)
  send_command("STARTTLS") do |resp|
    if resp.kind_of?(TaggedResponse) && resp.name == "OK"
      clear_cached_capabilities
      clear_responses
      start_tls_session
    end
  end
end