disconnect method
Disconnects from the mail service.
Also compare connect.
Implementation
Future<void> disconnect() async {
final futures = <Future>[
stopPollingIfNeeded(),
_incomingLock.synchronized(() => _incomingMailClient.disconnect()),
_outgoingLock.synchronized(() => _outgoingMailClient.disconnect()),
];
_isConnected = false;
await Future.wait(futures);
}