The [SmtpClient Class](http://msdn.microsoft.com/en-us/library/4971yhhc.aspx) states that instance members are not thread safe. This can be seen if concurrent calls are made to [Send](http://msdn.microsoft.com/en-us/library/swas0fwc.aspx) or [SendAsync](http://msdn.microsoft.com/en-us/library/x5x13z6h.aspx). Both methods will throw a InvalidOperationException on the second call if the first has not yet completed. 

The method [SendMailAsync](http://msdn.microsoft.com/en-us/library/hh193922.aspx), introduced in .NET 4.5, does not list InvalidOperationException as a thrown exception. Do the new .NET 4.5 methods implement some sort of queuing? Reflector isn't able to shed any light on the implementation details of this class, so I assume this has been implemented in native methods.

Can multiple threads call the SendMessageAsync method on a shared instance of the SMTP client safely?