The SmtpClient Class states that instance members are not thread safe. This can be seen if concurrent calls are made to Send or SendAsync. Both methods will throw a Invali
As a note (since I don't have enough points to comment), the traditional way to write an asynchronous operation was to use the Asynchronous Programming Model (APM), but today we typically use the Task-based Asynchronous Pattern (TAP) with its async/await keywords. And although it's not unusual to see TAP wrappers around APM methods, it's also possible to see APM wrappers around TAP methods.