supportsFlagging method
Determines if message flags such as \Seen can be stored.
POP3 servers do not support message flagging, for example. Note that even on POP3 servers the \Deleted / MessageFlags.deleted "flag" can be set. However, messages are really deleted and cannot be retrieved after marking them as deleted after the current POP3 session is closed.
Implementation
bool supportsFlagging() => _incomingMailClient.supportsFlagging();