.NET Framework 4.6 and 4.5
protected void OnSendCompleted(
Classes that inherit from the SmtpClient class can override the OnSendCompleted method to perform additional tasks when the SendCompleted event occurs.
OnSendCompleted also allows derived classes to handle SendCompleted without attaching a delegate. This is the preferred technique for handling SendCompleted in a derived class.Notes to Inheritors
When overriding OnSendCompleted in a derived class, be sure to call the base class’s OnSendCompleted method so that registered delegates receive the SendCompleted event.
Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0
.NET Framework Client Profile
Supported in: 4, 3.5 SP1