Get the UID of the message at the specified index.
Not all servers support UIDs, so you should first check the SupportsUids property.
/// is not a valid message index.
///
/// The has been disposed.
///
/// The is not connected.
///
/// The is not authenticated.
///
/// The mail spool does not support UIDs.
///
/// The operation was canceled via the cancellation token.
///
/// An I/O error occurred.
///
/// The command failed.
///
/// A protocol error occurred.
///