startTls method
Upgrades the current insure connection to SSL.
Opportunistic TLS (Transport Layer Security) refers to extensions in plain text communication protocols, which offer a way to upgrade a plain text connection to an encrypted (TLS or SSL) connection instead of using a separate port for encrypted communication.
Implementation
Future<GenericImapResult> startTls() async {
final cmd = Command('STARTTLS');
final response = await sendCommand<GenericImapResult>(
cmd, GenericParser(this, _selectedMailbox));
log('STARTTL: upgrading socket to secure one...', initial: 'A');
await upradeToSslSocket();
return response;
}