该触发条件有2种:
protected virtual Task OnDmtpClosing(
	ClosingEventArgs e
)Protected Overridable Function OnDmtpClosing ( 
	e As ClosingEventArgs
) As Taskabstract OnDmtpClosing : 
        e : ClosingEventArgs -> Task 
override OnDmtpClosing : 
        e : ClosingEventArgs -> Task